Semiconductor device, manufacturing method of the same, and electronic device

ABSTRACT

A semiconductor device in which parasitic capacitance is reduced is provided. A first insulating layer is deposited over a substrate. A first oxide insulating layer and an oxide semiconductor layer are deposited over the first insulating layer. A second oxide insulating layer is deposited over the oxide semiconductor layer and the first insulating layer. A second insulating layer and a first conductive layer are deposited over the second oxide insulating layer. A gate electrode layer, a gate insulating layer, and a third oxide insulating layer are formed by etching. A sidewall insulating layer including a region in contact with a side surface of the gate electrode layer is formed. A second conductive layer is deposited over the gate electrode layer, the sidewall insulating layer, the oxide semiconductor layer, and the first insulating layer. A third conductive layer is deposited over the second conductive layer. A low-resistance region is formed in the oxide semiconductor layer by performing heat treatment. An element contained in the second conductive layer moves from the second conductive layer to the oxide semiconductor layer side by performing the heat treatment. An element contained in the oxide semiconductor layer moves from the oxide semiconductor layer to the third conductive layer side by performing the heat treatment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to, for example, a semiconductor device, a display device, a light-emitting device, a power storage device, an imaging device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a semiconductor device or a method for manufacturing the semiconductor device.

In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A transistor and a semiconductor circuit are embodiments of semiconductor devices. In some cases, a memory device, a display device, or an electronic device includes a semiconductor device.

2. Description of the Related Art

A technique by which a transistor is formed using a semiconductor film formed over a substrate having an insulating surface has been attracting attention. The transistor is used in a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film that can be used for a transistor. As another material, an oxide semiconductor has been attracting attention.

For example, a transistor whose active layer includes an amorphous oxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn) is disclosed in Patent Document 1.

A method for manufacturing a transistor in which a conductive layer is formed over an oxide semiconductor layer and heat treatment is performed to reduce resistance of the oxide semiconductor layer are disclosed in Patent Document 2 and Non-Patent Document.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     2006-165528 -   [Patent Document 2] Japanese Published Patent Application No.     2013-175710

Non-Patent Document

-   [Non-Patent Document 1] 2015 Symposium on VLSI Technology Digest of     Technical Papers, T214-T215.

SUMMARY OF THE INVENTION

Manufacturing of semiconductor devices with highly integrated transistors requires miniaturization of transistors. However, in miniaturization of transistors, a remarkable increase in the parasitic capacitance of the transistors may cause a problem.

For example, in the case where parasitic capacitance exists in a channel (e.g., between a source electrode and a drain electrode) and the vicinity of the channel, a time for charging the parasitic capacitance is needed in the transistor operation; thus, not only the responsiveness of the transistor but the responsiveness of the semiconductor device is lowered.

It becomes more difficult to control various steps of manufacturing transistors (in particular, film formation, processing, and the like) as the miniaturization advances. Thus, variations due to a manufacturing process significantly affect transistor characteristics and reliability.

Thus, an object of one embodiment of the present invention is to reduce the parasitic capacitance in a transistor and in the vicinity of the transistor. Another object is to provide a semiconductor device capable of high-speed operation. Another object is to provide a semiconductor device with favorable electrical characteristics. Another object is to provide a semiconductor device with high reliability. Another object is to reduce variations in characteristics of a semiconductor device that are caused by a manufacturing process. Another object is to provide a semiconductor device including an oxide semiconductor layer having few oxygen vacancies. Another object is to provide a semiconductor device that can be manufactured in a simple process. Another object is to provide a semiconductor device in which the amount of interface states in the vicinity of the oxide semiconductor layer can be reduced. Another object is to provide a semiconductor device with low power consumption. Further, another object is to provide a novel semiconductor device or the like. Another object is to provide a manufacturing method of the semiconductor device.

Note that the description of these objects does not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

(1) One embodiment of the present invention is a method for manufacturing a semiconductor device including the steps of: depositing a first insulating layer over a substrate; sequentially depositing a first oxide insulating layer and a first oxide semiconductor layer over the first insulating layer; forming a second oxide insulating layer and a second oxide semiconductor layer by etching the first oxide insulating layer and the first oxide semiconductor layer into island shapes with a first mask; depositing a third oxide insulating layer over the second oxide semiconductor layer and the first insulating layer; depositing a second insulating layer over the third oxide insulating layer; depositing a first conductive layer over the second insulating layer; forming a gate electrode layer and a gate insulating layer by selectively etching the first conductive layer and the second insulating layer with a second mask; depositing a third insulating layer over the gate electrode layer, the second oxide semiconductor layer, and the first insulating layer; forming a sidewall insulating layer including a region in contact with a side surface of the gate electrode layer by performing dry etching on the third insulating layer; depositing a second conductive layer over the gate electrode layer, the sidewall insulating layer, the second oxide semiconductor layer, and the first insulating layer; depositing a third conductive layer over the second conductive layer; and forming a low-resistance region in the second oxide semiconductor layer by performing heat treatment. An element contained in the second conductive layer moves from the second conductive layer to the second oxide semiconductor layer side by performing the heat treatment. An element contained in the second oxide semiconductor layer moves from the second oxide semiconductor layer to the third conductive layer side by performing the heat treatment.

(2) One embodiment of the present invention is a method for manufacturing a semiconductor device including the steps of: depositing a first insulating layer over a substrate; sequentially depositing a first oxide insulating layer and a first oxide semiconductor layer over the first insulating layer; depositing a second oxide insulating layer and a second oxide semiconductor layer by performing etching on the first oxide insulating layer and the first oxide semiconductor layer into island shapes with a first mask; depositing a third oxide insulating layer over the second oxide semiconductor layer and the first insulating layer; depositing a second insulating layer over the third oxide insulating layer; forming a third insulating layer by performing planarization treatment on the second oxide insulating layer; forming a fourth insulating layer with a groove portion that reaches the third oxide insulating layer by etching part of the third insulating layer with a second mask; depositing a fifth insulating layer over the fourth insulating layer and the third oxide insulating layer; depositing a first conductive layer over the fifth insulating layer; forming a gate electrode layer and a sixth insulating layer by performing planarization treatment on the first conductive layer and the fifth insulating layer until the fourth insulating layer is exposed; forming a gate insulating layer by etching the fourth insulating layer and the sixth insulating layer with the gate electrode layer as a mask; depositing a seventh insulating layer over the gate electrode layer, the second oxide semiconductor layer, and the first insulating layer; forming a sidewall insulating layer including a region in contact with a side surface of the gate electrode layer by performing dry etching on the seventh insulating layer; depositing a second conductive layer over the gate electrode layer, the sidewall insulating layer, the second oxide semiconductor layer, and the first insulating layer; depositing a third conductive layer over the second conductive layer; and forming a low-resistance region in the second oxide semiconductor layer by performing beat treatment. An element contained in the second conductive layer moves from the second conductive layer to the second oxide semiconductor layer side by performing the heat treatment. An element contained in the second oxide semiconductor layer moves from the second oxide semiconductor layer to the third conductive layer side by performing the heat treatment.

(3) It is preferable that, in the above method for manufacturing a semiconductor device, the second conductive layer comprises cobalt, nickel, platinum, vanadium, chromium, or palladium, and the third conductive layer contains titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, rhodium, iridium, or cobalt.

(4) It is preferable that, in the above method for manufacturing a semiconductor device, the low-resistance region includes a region containing an alloy.

(5) It is preferable that, in the above method for manufacturing a semiconductor device, the heat treatment is performed at a temperature higher than or equal to 450° C. and lower than or equal to 650° C.

(6) Another embodiment of the present invention is a semiconductor device including a first insulating layer over a substrate; a first oxide insulating layer over the first insulating layer; an oxide semiconductor layer over the first oxide insulating layer; a second oxide insulating layer over the oxide semiconductor layer; a gate insulating layer over the second oxide insulating layer; a gate electrode layer over the gate insulating layer; and a sidewall insulating layer over the oxide semiconductor layer. A bottom surface of the oxide semiconductor layer is located above a bottom surface of the gate electrode layer in a cross section in a channel width direction. A top surface and a side surface of the oxide semiconductor layer each include a region facing the gate electrode layer. The sidewall insulating layer includes a region in contact with a side surface of the gate insulating layer. The oxide semiconductor layer includes first to third regions. The first region includes a region overlapping with the gate electrode layer. The second region includes a region which is between the first region and the third region and overlaps with the sidewall insulating layer. The second region includes a region with lower resistance than the first region. The third region includes a region with lower resistance than the second region. The second region and the third region each include any one of elements of cobalt, nickel, vanadium, and palladium, and any one of elements of titanium, zirconium, hafnium, and tantalum.

(7) It is preferable that, in the above semiconductor device, the second region and the third region each include a region containing an alloy.

(8) Another embodiment of the present invention is an electronic device including the semiconductor device, a housing, and a speaker.

According to one embodiment of the present invention, the parasitic capacitance in a transistor and in the vicinity of the transistor can be reduced, and a semiconductor device capable of high-speed operation can be provided. Alternatively, a semiconductor device with favorable electrical characteristics can be provided. Alternatively, a semiconductor device with high reliability can be provided. Alternatively, variations in characteristics of a transistor or a semiconductor device that are caused by a manufacturing process can be reduced. Alternatively, a semiconductor device including an oxide semiconductor layer having few oxygen vacancies can be provided. Alternatively, a semiconductor device that can be manufactured in a simple process can be provided. Alternatively, a semiconductor device with a structure in which the density of interface states in and near an oxide semiconductor layer can be reduced can be provided. Alternatively, a semiconductor device with low power consumption can be provided. Alternatively, a novel semiconductor device or the like can be provided. Alternatively, a method for manufacturing the semiconductor device can be provided.

Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily have all the effects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views which illustrate a transistor.

FIGS. 2A to 2C are a top view and cross-sectional views illustrating a transistor.

FIGS. 3A and 3B are a cross-sectional view of a transistor and a band diagram of an oxide semiconductor layer.

FIGS. 4A to 4D illustrate an ALD mechanism.

FIGS. 5A and 5B are schematic views of an ALD apparatus.

FIGS. 6A to 6C are a top view and cross-sectional views illustrating a method for manufacturing a transistor.

FIGS. 7A to 7C are a top view and cross-sectional views illustrating a method for manufacturing a transistor.

FIGS. 8A to 8C are a top view and cross-sectional views illustrating a method for manufacturing a transistor.

FIGS. 9A to 9C are a top view and cross-sectional views illustrating a method for manufacturing a transistor.

FIGS. 10A to 10C are a top view and cross-sectional views illustrating a method for manufacturing a transistor.

FIGS. 11A to 11C are a top view and cross-sectional views illustrating a method for manufacturing a transistor.

FIGS. 12A to 12C are a top view and cross-sectional views illustrating a method for manufacturing a transistor.

FIGS. 13A to 13C are a top view and cross-sectional views illustrating a method for manufacturing a transistor.

FIGS. 14A to 14C are a top view and cross-sectional views illustrating a method for manufacturing a transistor.

FIGS. 15A to 15C are a top view and cross-sectional views illustrating a method for manufacturing a transistor.

FIGS. 16A to 16C are a top view and cross-sectional views illustrating a method for manufacturing a transistor.

FIGS. 17A to 17C are a top view and cross-sectional views illustrating the method for manufacturing a transistor.

FIGS. 18A to 18C are a top view and cross-sectional views illustrating a method for manufacturing a transistor.

FIGS. 19A to 19C are a top view and cross-sectional views illustrating a transistor.

FIGS. 20A to 20C are a top view and cross-sectional views of a transistor.

FIGS. 21A to 21C are a top view and cross-sectional views of a transistor.

FIGS. 22A to 22C are a top view and cross-sectional views of a transistor.

FIGS. 23A to 23C are a top view and cross-sectional views of a transistor.

FIGS. 24A to 24E are Cs-corrected high-resolution TEM images of a cross section of a CAAC-OS and a cross-sectional schematic view of a CAAC-OS.

FIGS. 25A to 25E are Cs-corrected high-resolution TEM images of a plane of a CAAC-OS.

FIGS. 26A to 26D show structural analysis of a CAAC-OS and a single crystal oxide semiconductor layer by XRD.

FIGS. 27A and 27B show electron diffraction patterns of a CAAC-OS.

FIG. 28 shows a change in crystal part of an In—Ga—Zn oxide induced by electron irradiation.

FIGS. 29A to 29D are cross-sectional views and circuit diagrams of a semiconductor device.

FIGS. 30A to 30C are a cross-sectional view and circuit diagrams of a semiconductor device.

FIGS. 31A and 31B are plan views of an imaging device.

FIGS. 32A and 32B are plan views of pixels of an imaging device.

FIGS. 33A and 33B are cross-sectional views of an imaging device.

FIGS. 34A and 34B are cross-sectional views of an imaging device.

FIGS. 35A to 35C are circuit diagrams and a timing chart illustrating a semiconductor device of one embodiment of the present invention.

FIGS. 36A to 36C are a graph and circuit diagrams illustrating a semiconductor device of one embodiment of the present invention.

FIGS. 37A and 37B are a circuit diagram and a timing chart illustrating a semiconductor device of one embodiment of the present invention.

FIGS. 38A and 38B are a circuit diagram and a timing chart illustrating a semiconductor device of one embodiment of the present invention.

FIGS. 39A to 39E are a block diagram, circuit diagrams, and waveform diagrams for illustrating one embodiment of the present invention.

FIGS. 40A and 40B are a circuit diagram and a timing chart illustrating one embodiment of the present invention.

FIGS. 41A and 41B are circuit diagrams for illustrating one embodiment of the present invention.

FIGS. 42A to 42C are circuit diagrams each illustrating one embodiment of the present invention.

FIG. 43 illustrates a configuration example of an RF tag.

FIG. 44 illustrates a configuration example of a CPU.

FIG. 45 is a circuit diagram of a memory element.

FIGS. 46A to 46C illustrate a configuration example of a display device and circuit diagrams of pixels.

FIGS. 47A and 47B are a top view and a cross-sectional view of a liquid crystal display device.

FIGS. 48A and 48B are a top view and a cross-sectional view of a light-emitting device.

FIG. 49 illustrates a display module.

FIG. 50A is a perspective view illustrating a cross-sectional structure of a package using a lead frame interposer, and FIG. 50B is a plan view illustrating a structure of a module of a mobile phone.

FIGS. 51A to 51E illustrate electronic devices.

FIGS. 52A to 52D illustrate electronic devices.

FIGS. 53A to 53C illustrate electronic devices.

FIGS. 54A to 54F illustrate electronic devices.

FIG. 55 is a cross-sectional view illustrating a method for manufacturing a transistor.

FIG. 56 is a schematic cross-sectional view illustrating an evaluation sample.

FIGS. 57A and 57B illustrate cross-sectional STEM observation results of evaluation samples and EDX analysis results thereof.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings. Note that the present invention is not limited to the following description. It will be readily appreciated by those skilled in the art that modes and details of the present invention can be modified in various ways without departing from the spirit and scope of the present invention. The present invention therefore should not be construed as being limited to the following description of the embodiments. In structures of the invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and the description thereof is not repeated in some cases. The same components are denoted by different hatching patterns in different drawings, or the hatching patterns are omitted in some cases.

Notes on Description for Drawings

In this specification, terms for describing arrangement, such as “over” and “under,” are used for convenience to describe a positional relation between components with reference to drawings. Furthermore, the positional relation between components is changed as appropriate in accordance with a direction in which each component is described. Thus, there is no limitation on terms used in this specification, and description can be made appropriately depending on the situation.

The term “over” or “under” does not necessarily mean that a component is placed directly over or directly under and directly in contact with another component. For example, the expression “electrode B over insulating layer A” does not necessarily mean that the electrode B is over and in direct contact with the insulating layer A and can mean the case where another component is provided between the insulating layer A and the electrode B.

In this specification, trigonal and rhombohedral crystal systems are included in a hexagonal crystal system.

In drawings, the size, the layer thickness, or the region is determined arbitrarily for description convenience. Therefore, the size, the layer thickness, or the region is not limited to the illustrated scale. Note that the drawings are schematically shown for clarity, and embodiments of the present invention are not limited to shapes or values shown in the drawings.

In drawings such as plan views (also referred to as layout views) and perspective views, some of components might not be illustrated for clarity of the drawings.

Notes about the Description that can be Rephrased or Reworded

In this specification, in description of connections of a transistor, expressions “one of a source and a drain” (or a first electrode or a first terminal) and “the other of the source and the drain” (or a second electrode or a second terminal) are used. This is because a source and a drain of a transistor are interchangeable depending on the structure, operation conditions, or the like of the transistor. Note that the source or the drain of the transistor can also be referred to as a source (or drain) terminal, a source (or drain) electrode, or the like as appropriate depending on the situation.

In this specification, the term of “electrode” or “wiring” does not limit the function of components. For example, an “electrode” is sometimes used as part of a “wiring”, and vice versa. Furthermore, the term “electrode” or “wiring” can include the case where a plurality of “electrodes” or “wirings” is formed in an integrated manner.

In this specification, a transistor is an element having at least three terminals of a gate, a drain, and a source. The transistor includes a channel region between the drain (a drain terminal, a drain region, or a drain electrode) and the source (a source terminal, a source region, or a source electrode) and current can flow through the drain, the channel region, and the source.

Here, since the source and the drain of the transistor change depending on the structure, the operating condition, and the like of the transistor, it is difficult to define which is a source or a drain. Thus, a portion that functions as a source or a portion that functions as a drain is not referred to as a source or a drain in some cases. In that case, one of the source and the drain might be referred to as a first electrode, and the other of the source and the drain might be referred to as a second electrode.

Note that in this specification, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components, and the terms do not limit the components numerically.

In this specification, the terms “film” and “layer” can be interchanged depending on the case or circumstances. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. Also, the term “insulating film” can be changed into the term “insulating layer” in some cases.

The expression “being the same” in this specification may refer to having the same area or having the same shape. In addition, the expression “being the same” include a case of “being substantially the same” because a manufacturing process might cause some differences.

Note that what is described (or part thereof) in an embodiment in this specification can be applied to, combined with, or replaced with another content in the same embodiment and/or what is described (or part thereof) in another embodiment or other embodiments.

Note that by combining a diagram (or may be part of the diagram) illustrated in one embodiment in this specification with another part of the diagram, a different diagram (or may be part of the different diagram) illustrated in the embodiment, and/or a diagram (or may be part of the diagram) illustrated in one or a plurality of different embodiments, much more diagrams can be formed.

Notes on Definitions of Terms and Other Matters

The following are definitions of the terms mentioned in the above embodiments and other matters.

In this specification, the term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. A term “substantially parallel” indicates that the angle formed between two straight lines is greater than or equal to −30° and less than or equal to 30°. Furthermore, the term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly also includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. In addition, the term “substantially perpendicular” indicates that an angle formed between two straight lines is greater than or equal to 60° and less than or equal to 120°.

In this specification, the term “trench” or “groove” refers to a depression with a narrow belt shape.

Furthermore, in this specification, an explicit description “X and Y are connected” means that X and Y are electrically connected, X and Y are functionally connected, and X and Y are directly connected. Accordingly, another element may be provided between elements having a connection relation illustrated in drawings and texts, without being limited to a predetermined connection relation, for example, the connection relation illustrated in the drawings and the texts.

Here, each of X and Y denotes an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, or a layer).

Examples of the case where X and Y are directly connected include the case where an element that enables electrical connection between X and Y (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, a diode, a display element, a light-emitting element, or a load) is not connected between X and Y, and the case where X and Y are connected without the element that enables electrical connection between X and Y provided therebetween.

For example, in the case where X and Y are electrically connected, one or more elements that enable electrical connection between X and Y (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, a diode, a display element, a light-emitting element, or a load) can be connected between X and Y. Note that the switch is controlled to be turned on or off. That is, the switch is conducting or not conducting (is turned on or off) to determine whether current flows therethrough or not. Alternatively, the switch has a function of selecting and changing a current path. Note that the case where X and Y are electrically connected includes the case where X and Y are directly connected.

For example, in the case where X and Y are functionally connected, one or more circuits that enable functional connection between X and Y (e.g., a logic circuit such as an inverter, a NAND circuit, or a NOR circuit; a signal converter circuit such as a D/A converter circuit, an A/D converter circuit, or a gamma correction circuit; a potential level converter circuit such as a power supply circuit (e.g., a step-up circuit or a step-down circuit) or a level shifter circuit for changing the potential level of a signal; a voltage source; a current source; a switching circuit; an amplifier circuit such as a circuit that can increase signal amplitude, the amount of current, or the like, an operational amplifier, a differential amplifier circuit, a source follower circuit, or a buffer circuit; a signal generation circuit; a memory circuit; or a control circuit) can be connected between X and Y. Note that for example, in the case where a signal output from X is transmitted to Y even when another circuit is provided between X and Y, X and Y are functionally connected. The case where X and Y are functionally connected includes the case where X and Y are directly connected and X and Y are electrically connected.

Note that in this specification and the like, an explicit description “X and Y are electrically connected” means that X and Y are electrically connected (i.e., the case where X and Y are connected with another element or another circuit provided therebetween), X and Y are functionally connected (i.e., the case where X and Y are functionally connected with another circuit provided therebetween), and X and Y are directly connected (i.e., the case where X and Y are connected without another element or another circuit provided therebetween). That is, in this specification and the like, the explicit description “X and Y are electrically connected” is the same as the explicit description “X and Y are connected”.

For example, the case where a source (or a first terminal or the like) of a transistor is electrically connected to X through (or not through) Z1 and a drain (or a second terminal or the like) of the transistor is electrically connected to Y through (or not through) Z2, or the case where a source (or a first terminal or the like) of a transistor is directly connected to part of Z1 and another part of Z1 is directly connected to X while a drain (or a second terminal or the like) of the transistor is directly connected to part of Z2 and another part of Z2 is directly connected to Y, can be expressed by using any of the following expressions.

The expressions include, for example, “X, Y, a source (or a first terminal or the like) of a transistor, and a drain (or a second terminal or the like) of the transistor are electrically connected to each other, and X, the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are electrically connected to each other in that order”, “a source (or a first terminal or the like) of a transistor is electrically connected to X, a drain (or a second terminal or the like) of the transistor is electrically connected to Y, and X, the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are electrically connected to each other in that order”, and “X is electrically connected to Y through a source (or a first terminal or the like) and a drain (or a second terminal or the like) of a transistor, and X, the source (or the first terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor, and Y are connected in that order”. When the connection order in a circuit configuration is defined by an expression similar to the above examples, a source (or a first terminal or the like) and a drain (or a second terminal or the like) of a transistor can be distinguished from each other to specify the technical scope.

Other examples of the expressions include “a source (or a first terminal or the like) of a transistor is electrically connected to X through at least a first connection path, the first connection path does not include a second connection path, the second connection path is a path between the source (or the first terminal or the like) of the transistor and a drain (or a second terminal or the like) of the transistor, Z1 is on the first connection path, the drain (or the second terminal or the like) of the transistor is electrically connected to Y through at least a third connection path, the third connection path does not include the second connection path, and Z2 is on the third connection path”. It is also possible to use the expression “a source (or a first terminal or the like) of a transistor is electrically connected to X through at least Z1 on a first connection path, the first connection path does not include a second connection path, the second connection path includes a connection path through the transistor, a drain (or a second terminal or the like) of the transistor is electrically connected to Y through at least Z2 on a third connection path, and the third connection path does not include the second connection path”. Still another example of the expressions is “a source (or a first terminal or the like) of a transistor is electrically connected to X through at least Z1 on a first electrical path, the first electrical path does not include a second electrical path, the second electrical path is an electrical path from the source (or the first terminal or the like) of the transistor to a drain (or a second terminal or the like) of the transistor, the drain (or the second terminal or the like) of the transistor is electrically connected to Y through at least Z2 on a third electrical path, the third electrical path does not include a fourth electrical path, and the fourth electrical path is an electrical path from the drain (or the second terminal or the like) of the transistor to the source (or the first terminal or the like) of the transistor”. When the connection path in a circuit configuration is defined by an expression similar to the above examples, a source (or a first terminal or the like) and a drain (or a second terminal or the like) of a transistor can be distinguished from each other to specify the technical scope.

Note that these expressions are examples and there is no limitation on the expressions. Here, X, Y, Z1, and Z2 each denote an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, or a layer).

Even when independent components are electrically connected to each other in a circuit diagram, one component has functions of a plurality of components in some cases. For example, when part of a wiring also functions as an electrode, one conductive film functions as the wiring and the electrode. Thus, the term “electrical connection” in this specification also means such a case where one conductive film has functions of a plurality of components.

In this specification and the like, a structure in which a flexible printed circuit (FPC), a tape carrier package (TCP), or the like is attached to a substrate of a display panel, or a structure in which an integrated circuit (IC) is directly mounted on a substrate by a chip on glass (COG) method is referred to as a display device in some cases.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of the present invention and a manufacturing method of the semiconductor device will be described with reference to drawings.

<Structure of Transistor 10>

FIGS. 1A to 1C are a top view and cross-sectional views which illustrate a transistor 10 of one embodiment of the present invention. FIG. 1A is a top view and FIGS. 1B and 1C are cross-sectional views taken along dashed-dotted line A1-A2 and dashed-dotted line A3-A4 in FIG. 1A, respectively. In FIG. 1A, some components are scaled up or down or omitted for simplification of the drawing. In some cases, the direction of dashed-dotted line A1-A2 is referred to as a channel length direction, and the direction of dashed-dotted line A3-A4 is referred to as a channel width direction.

The transistor 10 includes a substrate 100, an insulating layer 110, an oxide insulating layer 121, an oxide semiconductor layer 122, an oxide insulating layer 123, a low-resistance region 127, a gate insulating layer 150, a gate electrode layer 160, a sidewall insulating layer 176, an insulating layer 180, and conductive layers 190 and 195.

The insulating layer 110 is provided over the substrate 100.

The oxide insulating layer 121 is provided over the insulating layer 110.

The oxide semiconductor layer 122 is provided over the oxide insulating layer 121.

The oxide insulating layer 123 is provided over the insulating layer 110 and the oxide semiconductor layer 122. The oxide insulating layer 123 preferably includes a region that is in contact with a side surface of the oxide semiconductor layer 122. Accordingly, the side surface of the oxide semiconductor layer 122 can be protected and thus the electrical characteristics of the transistor can be stabilized.

<Oxide Insulating Layer>

An oxide insulating layer (e.g., the oxide insulating layers 121 and 123) refers to a layer which basically has an insulating property and in which current can flow through the interface with a semiconductor and the vicinity thereof when a gate electric field or a drain electric field is increased.

The low-resistance region 127 includes a region in contact with the oxide semiconductor layer 122 and the oxide insulating layer 121. The low-resistance region 127 may be part of the oxide semiconductor layer 122 and/or the oxide insulating layer 121. The low-resistance region 127 contains any one of the elements of cobalt, nickel, vanadium, and palladium and any one of the elements of titanium, zirconium, hafnium, and tantalum. The low-resistance region 127 functions as a source and a drain.

Moreover, in the low-resistance region 127, the above material might form an alloy with the oxide semiconductor layer 122. The low-resistance region 127 can have lower resistance than the oxide semiconductor layer 122 by formation of the alloy.

The distance between a top surface of the low-resistance region 127 and a top surface of the substrate 100 may be the same as the distance between an upper end of the oxide semiconductor layer 122 and the top surface of the substrate 100 (see FIG. 2B), or may be larger or smaller than the distance between the upper end of the oxide semiconductor layer 122 and the top surface of the substrate 100.

In the above descriptions the low-resistance region 127 in the oxide semiconductor layer 122 can also be referred to as an oxide conductive layer.

Here, in the oxide semiconductor layer 122, a channel region overlapping with the gate electrode layer 160 is referred to as a first region, a region overlapping with the sidewall insulating layer 176 is referred to as a second region, and a region overlapping with neither the gate electrode layer 160 nor the sidewall insulating layer 176 is referred to as a third region. In that case, the low-resistance region 127 can be provided in part of the second region and the third region. The second region may be a lightly doped drain (LDD) region when compared with the third region. Alternatively, the low-resistance region 127 might reach the oxide insulating layer 121.

In the above, the second region has a region with lower resistance than the first region, and the third region has a region with lower resistance than the second region. The resistance can be expressed by the resistance (e.g., sheet resistance) measurement or the impurity concentration.

The gate insulating layer 150 is provided over the oxide insulating layer 123.

The gate electrode layer 160 is provided over the gate insulating layer 150. Note that the gate electrode layer 160, the gate insulating layer 150, the oxide insulating layer 123, and the oxide semiconductor layer 122 are overlapped with each other.

The sidewall insulating layer 176 is provided over the insulating layer 110 and the low-resistance region 127 and includes a region that is in contact with side surfaces of the gate insulating layer 150 and the gate electrode layer 160.

The insulating layer 180 is provided over the insulating layer 110.

The conductive layer 190 is provided over the low-resistance region 127. The conductive layer 190 includes a region electrically connected to the low-resistance region 127.

The conductive layer 195 is provided over the conductive layer 190.

With the above structure, parasitic capacitance between a gate and the source or parasitic capacitance between the gate and the drain can be reduced. Thus, the cutoff frequency characteristics of the transistor 10 are improved, for example, and therefore the transistor can operate at high speed.

Furthermore, the gate, the source, and the drain of the transistor 10 can be formed in a self-aligned manner; thus, alignment accuracy can be improved and miniaturized transistors can be easily manufactured.

When seen in the channel width direction, the transistor 10 includes a region where the gate electrode layer 160 faces the side surfaces of the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 with the gate insulating layer 150 provided therebetween as illustrated in the cross-sectional view in FIG. 1C, which is taken along line A3-A4. That is, the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 are surrounded by the electric field of the gate electrode layer 160 in the channel width direction when voltage is applied to the gate electrode layer 160. The transistor structure in which a semiconductor is surrounded by the electric field of the gate electrode layer 160 is referred to as a surrounded channel (s-channel) structure. Moreover, in the s-channel structure, the bottom surface of the oxide semiconductor layer 122 is located at a higher level than that of the gate electrode layer 160.

Here, the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 are collectively referred to as an oxide. When the transistor 10 is in an on state, a channel is formed in the entire oxide semiconductor layer 122 (bulk), so that the on-state current is increased. When the transistor is in an off state, the channel region formed in the oxide semiconductor layer 122 with a wide band gap serves as a potential barrier; as a result, the off-state current can be further reduced.

<Channel Length>

Note that the channel length of a transistor refers to, for example, a distance between a source (a source region or a source electrode) and a drain (a drain region or a drain electrode) in a region where a semiconductor (or a portion where current flows in a semiconductor when the transistor is on) and a gate electrode overlap with each other or a region where a channel is formed in a top view of the transistor. In one transistor, channel lengths in all regions do not necessarily have the same value. In other words, the channel length of one transistor is not fixed to one value in some cases. Therefore, in this specification, the channel length is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed.

<Channel Width>

Note that the channel width refers to, for example, the length of a region where a semiconductor (or a portion where current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other. In one transistor, channel widths in all regions do not necessarily have the same value. In other words, the channel width of one transistor is not fixed to one value in some cases. Therefore, in this specification, the channel width is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed.

Note that depending on the transistor structure, a channel width in a region where a channel is actually formed (hereinafter referred to as an effective channel width) is different from a channel width shown in a top view of a transistor (hereinafter referred to as an apparent channel width) in some cases. For example, in a transistor having a three-dimensional structure, an effective channel width is larger than an apparent channel width shown in a top view of the transistor, and its influence cannot be ignored in some cases. For example, in a miniaturized transistor having a three-dimensional structure, the proportion of a channel region formed in a side surface of a semiconductor is high in some cases. In that case, an effective channel width obtained when a channel is actually formed is greater than an apparent channel width shown in the top view.

In a transistor having a three-dimensional structure, an effective channel width is difficult to measure in some cases. For example, estimation of an effective channel width from a design value requires an assumption that the shape of a semiconductor is known. Therefore, without accurate information on the shape of a semiconductor, it is difficult to measure an effective channel width accurately.

<SCW>

Therefore, in this specification, in a top view of a transistor, an apparent channel width in a region where a semiconductor and a gate electrode overlap with each other is referred to as a surrounded channel width (SCW) in some cases. Further, in this specification, in the case where the term “channel width” is simply used, it may denote a surrounded channel width or an apparent channel width. Alternatively, in this specification, in the case where the term “channel width” is simply used, it may denote an effective channel width in some cases. Note that the values of a channel length, a channel width, an effective channel width, an apparent channel width, a surrounded channel width, and the like can be determined by obtaining and analyzing a cross-sectional TEM image and the like.

Note that in the case where field-effect mobility, a current value per channel width, and the like of a transistor are obtained by calculation, a surrounded channel width may be used for the calculation. In that case, a value different from the value obtained by calculation using an effective channel width is obtained in some cases.

<Improvement of Characteristics in Miniaturization>

High integration of a semiconductor device requires miniaturization of a transistor. However, it is known that miniaturization of a transistor causes deterioration of electrical characteristics of the transistor. A decrease in channel width causes a reduction in on-state current.

In the transistor of one embodiment of the present invention illustrated in FIGS. 1A to 1C, for example, as described above, the oxide insulating layer 123 is formed so as to cover the oxide semiconductor layer 122 where a channel is forming and the channel formation region and the gate insulating layer are not in contact with each other. Accordingly, scattering of carriers at the interface between the channel formation region and the gate insulating layer can be reduced and the on-state current of the transistor can be increased.

In the transistor of one embodiment of the present invention, the gate electrode layer 160 is formed to electrically surround the oxide semiconductor layer 122, which is to be a channel, in the channel width direction; accordingly, a gate electric field is applied to the oxide semiconductor layer 122 in the side surface direction in addition to the perpendicular direction. Furthermore, the bottom surface of the oxide semiconductor layer 122 is located at a higher level than that of the gate electrode layer 160 and thus a gate electric field is also applied to the bottom surface of the oxide semiconductor layer 122. In other words, a gate electric field is applied to the oxide semiconductor layer 122 entirely, so that current flows in the whole of the oxide semiconductor layer 122, leading to a further increase in on-state current.

In the transistor of one embodiment of the present invention, the oxide insulating layer 123 is formed over the oxide insulating layer 121 and the oxide semiconductor layer 122, so that an interface state is unlikely to be formed. In addition, impurities do not enter the oxide semiconductor layer 122 from above and below because the oxide semiconductor layer 122 is positioned at the middle. Therefore, the transistor can achieve not only the increase in the on-state current but also stabilization of the threshold voltage and a reduction in the S value (subthreshold value). Thus, a drain current at a gate voltage VG of 0 V can be reduced and power consumption can be reduced. Further, since the threshold voltage of the transistor becomes stable, long-term reliability of the semiconductor device can be improved.

In the transistor of one embodiment of the present invention, the gate electrode layer 160 is formed to electrically surround the oxide semiconductor layer 122, which is to be a channel, in the channel width direction; accordingly, a gate electric field is applied to the oxide semiconductor layer 122 in the side surface direction in addition to the perpendicular direction. That is, a gate electric field is applied to the entire oxide semiconductor layer 122, so that the influence of a drain electric field can be reduced and a short-channel effect can be significantly suppressed. Therefore, the transistor can have favorable characteristics even when miniaturized.

Alternatively, when the transistor of one embodiment of the present invention includes a wide band gap material as the oxide semiconductor layer 122, which is to be the channel, the transistor can have high source-drain breakdown voltage and stable electrical characteristics in various temperature environments.

Although an example where a channel or the like is formed in an oxide semiconductor layer or the like is described in this embodiment, one embodiment of the present invention is not limited thereto. For example, depending on circumstances or conditions, a channel, the vicinity of the channel, a source region, a drain region, or the like may be formed using a material containing silicon (including strained silicon), germanium, silicon germanium, silicon carbide, gallium arsenide, aluminum gallium arsenide, indium phosphide, gallium nitride, an organic semiconductor, or the like.

<Components of Transistor>

Components of a transistor of this embodiment will be described below.

<Substrate 100>

A glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or the like can be used as the substrate 100. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon or silicon carbide, a compound semiconductor substrate of silicon germanium, a silicon on insulator (SOI) substrate, or the like can be used. Still alternatively, any of these substrates provided with a semiconductor element may be used. The substrate 100 is not limited to a simple supporting substrate, and may be a substrate where a device such as a transistor is formed. In that case, any one or more of the gate, the source, and the drain of the transistor may be electrically connected to the device.

Alternatively, a flexible substrate may be used as the substrate 100. As a method for providing the transistor over a flexible substrate, there is a method in which the transistor is formed over a non-flexible substrate and then the transistor is separated and transferred to the substrate 100 that is a flexible substrate. In that case, a separation layer is preferably provided between the non-flexible substrate and the transistor. As the substrate 100, a sheet, a film, or a foil containing a fiber may be used, for example. The substrate 100 may have elasticity. The substrate 100 may have a property of returning to its original shape when bending or pulling is stopped. Alternatively, the substrate 100 may have a property of not returning to its original shape. The thickness of the substrate 100 is, for example, greater than or equal to 5 lint and less than or equal to 700 μm, preferably greater than or equal to 10 μm and less than or equal to 500 μm and further preferably greater than or equal to 15 μm and less than or equal to 300 μm. When the substrate 100 has a small thickness, the weight of the semiconductor device can be reduced. When the substrate 100 has a small thickness, even in the case of using glass or the like, the substrate 100 might have elasticity or a property of returning to its original shape when bending or pulling is stopped. Therefore, an impact applied to the semiconductor device over the substrate 100, which is caused by dropping or the like, can be reduced. That is, a durable semiconductor device can be provided.

For the substrate 100 which is a flexible substrate, metal, an alloy, resin, glass, or fiber thereof can be used, for example. The flexible substrate 100 preferably has a lower coefficient of linear expansion because deformation due to an environment is suppressed. The flexible substrate 100 is formed using, for example, a material whose coefficient of linear expansion is lower than or equal to 1×10⁻³/K, lower than or equal to 5×10⁻⁵/K, or lower than or equal to 1×10⁻⁵/K. Examples of the resin include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, acrylic, and polytetrafluoroethylene (PTFE). In particular, aramid is preferably used for the flexible substrate 100 because of its low coefficient of linear expansion.

<Insulating Layer 110>

As the insulating layer 110, an insulating film containing one or more of silicon (Si), nitrogen (N), oxygen (O), fluorine (F), hydrogen (H), aluminum (Al), gallium (Ga), germanium (Ge), yttrium (Y), zirconium (Zr), lanthanum (La), neodymium (Nd), hafnium (HO, and tantalum (Ta) can be used.

The insulating layer 110 can have a function of supplying oxygen to the oxide semiconductor layer 122 as well as a function of preventing diffusion of impurities from the substrate 100. For this reason, the insulating layer 110 is preferably an insulating film containing oxygen, further preferably an insulating film having an oxygen content higher than that in the stoichiometric composition. For example, the insulating layer 110 is a film in which the amount of released oxygen converted into oxygen atoms is 1.0×10¹⁹ atoms/cm³ or more in IDS analysis. Note that the temperature of the film surface in the TDS analysis is preferably higher than or equal to 100° C. and lower than or equal to 700° C., or higher than or equal to 100° C. and lower than or equal to 500° C. In the case where the substrate 100 is provided with another device as described above, the insulating layer 110 also functions as an interlayer insulating film. In that case, the insulating layer 110 is preferably subjected to planarization treatment such as chemical mechanical polishing (CMP) treatment so as to have a flat surface.

When the insulating layer 110 contains fluorine, fluorine gasified from the insulating layer can stabilize an oxygen vacancy in the oxide semiconductor layer 122.

<Oxide Insulating Layer 121, Oxide Semiconductor Layer 122, and Oxide Insulating Layer 123>

The oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 are oxide semiconductor films containing In or Zn and typically contain an In—Ga oxide, an In—Zn oxide, an In—Mg oxide, a Zn—Mg oxide, or an In-M-Zn oxide (M is Al, Ti, Ga, Y, Sn, Zr, La, Ce, Mg, Hf, Fe, or Nd).

An oxide that can be used for each of the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 preferably contains at least indium (In) or zinc (Zn). Alternatively, both In and Zn are preferably contained. In order to reduce variations in electrical characteristics of the transistors including the oxide, the oxide preferably contains a stabilizer in addition to In and Zn.

As examples of a stabilizer, gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al), zirconium (Zr), and the like can be given. As another example of stabilizer, lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) can be given.

The indium and gallium contents in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 can be compared with each other by time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectrometry (XPS), or inductively coupled plasma mass spectrometry (ICP-MS).

Since the oxide semiconductor layer 122 has an energy gap of 2 eV or more, preferably 2.5 eV or more and further preferably 3 eV or more, the off-state current of the transistor 10 can be low.

The thickness of the oxide semiconductor layer 122 is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm and further preferably greater than or equal to 3 nm and less than or equal to 50 nm.

The thickness of the oxide semiconductor layer 122 may be larger than, equal to, or smaller than that of the oxide insulating layer 121. If the thickness of the oxide semiconductor layer 122 is large, the on-state current of the transistor can be increased. The thickness of the oxide insulating layer 121 may be determined as appropriate as long as formation of an interface state at the interface with the oxide semiconductor layer 122 can be suppressed. For example, the thickness of the oxide semiconductor layer 122 is larger than that of the oxide insulating layer 121, preferably 2 or more times, further preferably 4 or more times, and still further preferably 6 or more times, as large as that of the oxide insulating layer 121. In the case where there is no need to increase the on-state current of the transistor, the thickness of the oxide insulating layer 121 may be larger than or equal to that of the oxide semiconductor layer 122.

In the case where the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 have different compositions from one another, the interfaces thereof can be observed with a scanning transmission electron microscope (STEM) in some cases.

The content of In in the oxide semiconductor layer 122 is preferably higher than those in the oxide insulating layers 121 and 123. In an oxide semiconductor layer, the s orbital of heavy metal mainly contributes to carrier transfer, and when the proportion of In in the oxide semiconductor is increased, overlap of the s orbitals is likely to be increased. Therefore, an oxide having a composition in which the proportion of In is higher than that of M has higher mobility than an oxide having a composition in which the proportion of In is equal to or lower than that of M. Thus, with the use of an oxide having a high indium content for the oxide semiconductor layer 122, a transistor having high field-effect mobility can be obtained.

In the case where the oxide semiconductor layer 122 is an In-M-Zn oxide (M is Al, Ti, Ga, Y, Sn, Zr, La, Ce, Mg, Hf, Fe, or Nd) and a target having the atomic ratio of metal elements of In:M:Zn=x₂:y₂:z₂ is used for depositing the oxide semiconductor layer 122 by a sputtering method, x₂/(x₂+y₂+z₂) is preferably greater than or equal to ⅓. Furthermore, x₂/y₂ is preferably greater than or equal to ⅓ and less than or equal to 10 and further preferably greater than or equal to 1 and less than or equal to 6, and z₂/y₂ is preferably greater than or equal to ⅓ and less than or equal to 10 and further preferably greater than or equal to 1 and less than or equal to 6. In this manner, a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film is easily formed as the oxide semiconductor layer 122. Typical examples of the atomic ratio of metal elements of the target include In:M:Zn=1:1:1, 1:1:1.2, 2:1:1.5, 2:1:2.3, 2:1:3, 3:1:2, 4:2:3, 4:2:4.1, 5:1:6, and 5:1:7.

When the atomic ratio of Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, Hf, Fe, or Nd having a function as a stabilizer is higher than that of In in each of the oxide insulating layers 121 and 123, any of the following effects might be obtained.

(1) The energy gap of each of the oxide insulating layers 121 and 123 is widened.

(2) The electron affinity of each of the oxide insulating layers 121 and 123 is reduced.

(3) Impurities from the outside are blocked.

(4) An insulating property of each of the oxide insulating layers 121 and 123 is higher than that of the oxide semiconductor layer 122.

(5) Oxygen vacancies are less likely to be generated in the oxide insulating layers 121 and 123 each containing a larger amount of Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, Hf, Fe, or Nd in an atomic ratio than the amount of In in an atomic ratio because Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, Hf, Fe, and Nd are metal elements that can be strongly bonded to oxygen.

By providing an oxide film in which oxygen vacancies are less likely to be generated than in the oxide semiconductor layer 122, over and under and in contact with the oxide semiconductor layer 122, oxygen vacancies in the oxide semiconductor layer 122 can be reduced. Further, since the oxide semiconductor layer 122 is in contact with the oxide insulating layers 121 and 123 containing one or more metal elements forming the oxide semiconductor layer 122, the density of interface states at the interface between the oxide insulating layer 121 and the oxide semiconductor layer 122 and at the interface between the oxide semiconductor layer 122 and the oxide insulating layer 123 is extremely low. Therefore, after oxygen is added to the oxide insulating layer 121, the oxide insulating layer 123, the gate insulating layer 150, the insulating layer 110, or the insulating layer 180, the oxygen is transferred through the oxide insulating layers 121 and 123 to the oxide semiconductor layer 122 by heat treatment; however, the oxygen is hardly trapped by the interface states at this time, and the oxygen in the oxide insulating layer 121 or 123 can be efficiently transferred to the oxide semiconductor layer 122. Accordingly, oxygen vacancies in the oxide semiconductor layer 122 can be reduced. Since oxygen is added to the oxide insulating layers 121 and 123, oxygen vacancies in the oxide insulating layers 121 and 123 can be reduced. In other words, the density of localized states of at least the oxide semiconductor layer 122 can be reduced.

In addition, when the oxide semiconductor layer 122 is in contact with an insulating film including a different constituent element (e.g., a gate insulating layer including a silicon oxide film), an interface state is sometimes formed and the interface state forms a channel. At this time, a second transistor having a different threshold voltage appears, so that an apparent threshold voltage of the transistor is varied. However, since the oxide insulating layers 121 and 123 containing one or more kinds of metal elements forming the oxide semiconductor layer 122 are in contact with the oxide semiconductor layer 122, an interface state is not easily formed at the interface between the oxide insulating layer 121 and the oxide semiconductor layer 122 and at the interface between the oxide insulating layer 123 and the oxide semiconductor layer 122.

In this embodiment, the amount of oxygen vacancies in the oxide semiconductor layer 122, and further the amount of oxygen vacancies in the oxide insulating layers 121 and 123 in contact with the oxide semiconductor layer 122 can be reduced; thus, the density of localized states of the oxide semiconductor layer 122 can be reduced. As a result, the transistor 10 in this embodiment has small variations in threshold voltage and high reliability. Further, the transistor 10 of this embodiment has excellent electrical characteristics.

In the case where the gate insulating layer 150 and the oxide semiconductor layer 122 are in contact with each other and a channel is formed at the interface therebetween, interface scattering occurs at the interface and the field-effect mobility of the transistor is decreased. However, since the oxide insulating layers 121 and 123 containing one or more kinds of metal elements forming the oxide semiconductor layer 122 are provided in contact with the oxide semiconductor layer 122, scattering of carriers does not easily occur at the interface between the oxide semiconductor layer 122 and the oxide insulating layer 121 and at the interface between the oxide semiconductor layer 122 and the oxide insulating layer 123, and thus the field-effect mobility of the transistor can be increased.

Each of the oxide insulating layers 121 and 123 is typically an In—Ga oxide, an In—Zn oxide, an In—Mg oxide, a Ga—Zn oxide, a Zn—Mg oxide, or an In-M-Zn oxide (M is Al, Ti, Ga, Y, Sn, Zr, La, Ce, Mg, Hf, Fe, or Nd), and has the energy level at the conduction band minimum that is closer to a vacuum level than the energy level at the conduction band minimum of the oxide semiconductor layer 122 is. Typically, a difference between the energy level at the conduction band minimum of the oxide semiconductor layer 122 and the energy level at the conduction band minimum of each of the oxide insulating layers 121 and 123 is greater than or equal to 0.05 eV, greater than or equal to 0.07 eV, greater than or equal to 0.1 eV, or greater than or equal to 0.2 eV and also less than or equal to 2 eV, less than or equal to 1 eV, less than or equal to 0.5 eV, or less than or equal to 0.4 eV. That is, the difference between the electron affinity of the oxide semiconductor layer 122 and the electron affinity of each of the oxide insulating layers 121 and 123 is greater than or equal to 0.05 eV, greater than or equal to 0.07 eV, greater than or equal to 0.1 eV, or greater than or equal to 0.2 eV and also less than or equal to 2 eV, less than or equal to 1 eV, less than or equal to 0.5 eV, or less than or equal to 0.4 eV. Note that the electron affinity refers to a difference between the vacuum level and the energy level at the conduction band minimum.

In the case where the oxide insulating layers 121 and 123 are In-M-Zn oxides (M is Al, Ti, Ga, Y, Sn, Zr, La, Ce, Mg, Hf, Fe, or Nd), the oxide insulating layers 121 and 123 have a higher atomic ratio of M (Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, Hf, Fe, or Nd) than the oxide semiconductor layer 122, and the element represented by M is more strongly bonded to oxygen than indium is; thus, generation of oxygen vacancies in the oxide insulating layers 121 and 123 can be suppressed. That is, the oxide insulating layers 121 and 123 are oxide semiconductor films in which oxygen vacancies are less likely to be generated than in the oxide semiconductor layer 122.

In the case where the oxide insulating layer 121 is an In-M-Zn oxide (M is Al, Ti, Ga, Y, Sn, Zr, La, Ce, Mg, Hf, Fe, or Nd) and a target having the atomic ratio of metal elements of In:M:Zn=x₁:y₁:z₁ is used for depositing the oxide insulating layer 121, x₁/y₁ is preferably less than z₁/y₁, and z₁/y₁ is preferably greater than or equal to 0.1 and less than or equal to 6 and further preferably greater than or equal to 0.2 and less than or equal to 3.

In the case where the oxide insulating layer 123 is an In-M-Zn oxide (M is Al, Ti, Ga, Y, Sn, Zr, La, Ce, Mg, Hf, Fe, or Nd) and a target having the atomic ratio of metal elements of In:M:Zn=x₃:y₃:z₃ is used for depositing the oxide insulating layers 121 and 123 by a sputtering method, x₃/y₃ is preferably less than x₂/y₂, and z₃/y₃ is preferably greater than or equal to ⅓ and less than or equal to 6 and further preferably greater than or equal to 1 and less than or equal to 6. Note that when z₃/y₃ is greater than or equal to 1 and less than or equal to 6, CAAC-OS films are easily formed as the oxide insulating layer 121 and the oxide insulating layer 123.

Typical examples of the atomic ratio between the metal elements in a target used for depositing the oxide insulating layer 121 and the oxide insulating layer 123 by a sputtering method include In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:6, In:M:Zn=1:3:8, In:M:Zn=1:4:4, In:M:Zn=1:4:5, In:M:Zn=1:4:6, In:M:Zn=1:4:7, In:M:Zn=1:4:8, In:M:Zn=1:5:5, In:M:Zn=1:5:6, In:M:Zn=1:5:7, In:M:Zn=1:5:8, In:M:Zn=1:6:8, In:M:Zn=1:6:4, In:M:Zn=1:9:6, In:M:Zn=1:10:1, and Ga:Zn=10:1. The atomic ratio is not limited to the above, and the atomic ratio may be appropriately set in accordance with needed semiconductor characteristics.

Since the oxide insulating layers 121 and 123 have higher insulating properties than the oxide semiconductor layer 122, they each can have a function of a gate insulating layer.

Alternatively, the oxide insulating layer 123 can be metal oxide, such as aluminum oxide (AlO_(x)), gallium oxide (GaO_(x)), hafnium oxide (HfO_(x)), silicon oxide (SiO_(x)), germanium oxide (GeO_(x)), or zirconia oxide (ZrO_(x)); or the metal oxide may be provided over the oxide insulating layer 123.

The thickness of the oxide insulating layer 123 may be determined as appropriate as long as formation of an interface state at the interface with the oxide semiconductor layer 122 is inhibited. For example, the thickness of the oxide insulating layer 123 may be set smaller than or equal to that of the oxide insulating layer 121. If the thickness of the oxide insulating layer 123 is large, it might become difficult for the electric field from the gate electrode layer 160 to reach the oxide semiconductor layer 122. For this reason, the thickness of the oxide insulating layer 123 is preferably small. For example, the thickness of the oxide insulating layer 123 is smaller than that of the oxide semiconductor layer 122. Note that the thickness of the oxide insulating layer 123 is not limited to the above, and may be determined as appropriate in accordance with the driving voltage of the transistor in consideration of the withstand voltage of the gate insulating layer 150.

For example, the thickness of the oxide insulating layer 123 is preferably greater than or equal to 1 nm and less than or equal to 20 inn or greater than or equal to 3 nm and less than or equal to 10 nm.

In each of the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123, the proportion of each atom in the above-described atomic ratio varies within a range of ±40% as an error in some cases.

For example, when an oxide semiconductor film to be the oxide semiconductor layer 122 is deposited by a sputtering method using a target in which the atomic ratio of the metal elements is In:Ga:Zn=1:1:1, the atomic ratio of the metal elements of the oxide semiconductor film to be the oxide semiconductor layer 122 is approximately In:Ga:Zn=1:1:0.6, which means that the atomic ratio of zinc is not changed or reduced in some cases. Therefore, the atomic ratio described in this specification includes the atomic ratio in vicinity thereof.

<Hydrogen Concentration>

Hydrogen contained in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 reacts with oxygen bonded to a metal atom to be water, and in addition, an oxygen vacancy is formed in a lattice from which oxygen is released (or a portion from which oxygen is released). An electron serving as a carrier can be generated due to entry of hydrogen into the oxygen vacancy or due to bonding of part of hydrogen to oxygen bonded to a metal atom. Thus, a transistor including an oxide semiconductor layer which contains hydrogen is likely to be normally on.

Accordingly, it is preferable that hydrogen be reduced as much as possible as well as the oxygen vacancies in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 and at the interfaces between the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123. The concentrations of hydrogen in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 and at the interfaces between the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123, which are obtained by secondary ion mass spectrometry (SIMS), are desirably higher than or equal to 1×10¹⁶ atoms/cm³ and lower than or equal to 2×10²⁰ atoms/cm³, preferably higher than or equal to 1×10¹⁶ atoms/cm³ and lower than or equal to 5×10¹⁹ atoms/cm³, further preferably higher than or equal to 1×10¹⁶ atoms/cm³ and lower than or equal to 1×10¹⁹ atoms/cm³, and still further preferably higher than or equal to 1×10¹⁶ atoms/cm³ and lower than or equal to 5×10¹⁸ atoms/cm³. As a result, the transistor 10 can have positive threshold voltage (normally-off characteristics).

<Concentrations of Carbon and Silicon>

When silicon and carbon, which are elements belonging to Group 14, are contained in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 and at the interfaces between the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123, oxygen vacancies are increased and an n-type region is formed in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 in some cases. It is therefore preferable to reduce the concentrations of silicon and carbon in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 and at the interfaces between the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123. The concentrations of silicon and carbon in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 and at the interfaces between the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123, which are obtained by SIMS, are desirably higher than or equal to 1×10¹⁶ atoms/cm³ and lower than or equal to 1×10¹⁹ atoms/cm³, preferably higher than or equal to 1×10¹⁶ atoms/cm³ and lower than or equal to 5×10¹⁸ atoms/cm³ and further preferably higher than or equal to 1×10¹⁶ atoms/cm³ and lower than or equal to 2×10¹⁸ atoms/cm³. As a result, the transistor 10 can have positive threshold voltage.

<Concentration of Alkali Metal>

Alkali metal and alkaline earth metal can generate carriers when bonded to an oxide semiconductor, which can increase the off-state current of the transistor. It is thus preferable to reduce the concentrations of alkali metal and alkaline earth metal in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 and at the interfaces between the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123. For example, the concentrations of alkali metal and alkaline earth metal in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 and at the interfaces between the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123, which are obtained by SIMS, are desirably lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁶ atoms/cm³. As a result, the transistor 10 can have positive threshold voltage.

<Concentration of Nitrogen>

When nitrogen is contained in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 and at the interfaces between the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123, an electron serving as a carrier is generated and accordingly carrier density is increased, so that n-type regions are formed in some cases. Thus, a transistor including an oxide semiconductor layer which contains nitrogen is likely to be normally on. Thus, it is preferable that nitrogen be reduced as much as possible in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 and at the interfaces between the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123. For example, the concentrations of nitrogen in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 and at the interfaces between the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123, which are obtained by SIMS, are preferably higher than or equal to 1×10¹⁵ atoms/cm³ and lower than or equal to 5×10¹⁹ atoms/cm³, further preferably higher than or equal to 1×10¹⁵ atoms/cm³ and lower than or equal to 5×10¹⁸ atoms/cm³, still further preferably higher than or equal to 1×10¹⁵ atoms/cm³ and lower than or equal to 1×10¹⁸ atoms/cm³, and yet still further preferably higher than or equal to 1×10¹⁵ atoms/cm³ and lower than or equal to 5×10¹⁷ atoms/cm³. As a result, the transistor 10 can have positive threshold voltage.

However, in the case where excess zinc exists in the oxide semiconductor layer 122, the concentrations of nitrogen are not limited to the above range. Excess zinc might cause oxygen vacancies in the oxide semiconductor layer 122, and when the oxide semiconductor layer 122 containing excess zinc also contains nitrogen at 0.001 atomic % to 3 atomic %, the oxygen vacancies caused by the excess zinc can be inactivated in some cases. Therefore, the nitrogen can reduce variations in transistor characteristics and can improve the reliability.

<Carrier Density>

The carrier densities of the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 can be lowered by reduction in impurities in the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123. The carrier densities of the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 are less than or equal to 1×10¹⁵/cm³, preferably less than or equal to 1×10¹³/cm³, further preferably less than 8×10¹¹/cm³, still further preferably less than 1×10¹¹/cm³, and yet still further preferably less than 1×10¹⁰/cm³, and is greater than or equal to 1×10⁻⁹/cm³.

As described above, when an oxide having a low impurity concentration and a low density of defect states is used as each of the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123, the transistor can have more excellent electrical characteristics. Here, the state in which the impurity concentration is low and the density of defect states is low (the amount of oxygen vacancies is small) is described as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide has few carrier generation sources, and thus can have a low carrier density in some cases. Thus, a transistor including the oxide in which a channel region is formed is likely to have positive threshold voltage. A highly purified intrinsic or substantially highly purified intrinsic oxide has a low density of defect states and accordingly has a low density of trap states in some cases. Further, a highly purified intrinsic or substantially highly purified intrinsic oxide has an extremely low off-state current; the off-state current can be lower than or equal to the measurement limit of a semiconductor parameter analyzer, i.e., lower than or equal to 1×10⁻¹³ A, at a voltage between a source electrode and a drain electrode (drain voltage) of from 1 V to 10 V. Thus, the transistor whose channel region is formed in the oxide can have a small variation in electrical characteristics and high reliability.

A transistor in which a highly purified oxide is used for a channel formation region exhibits extremely low off-state current. For example, in the case where the voltage between the source and the drain is set to approximately 0.1 V, 5 V, or 10 V, the off-state current standardized on the channel width of the transistor can be as low as several yoctoamperes per micrometer to several zeptoamperes per micrometer.

The oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 may have a non-single crystal structure, for example. The non-single crystal structure includes a CAAC-OS which is described later, a polycrystalline structure, a microcrystalline structure, or an amorphous structure, for example. Among the non-single crystal structures, the amorphous structure has the highest density of defect states, whereas the CAAC-OS has the lowest density of defect states.

The oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 may have a microcrystalline structure, for example. In some cases, the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 which have the microcrystalline structure each include a microcrystal with a size greater than or equal to 1 nm and less than 10 nm, for example. Alternatively, the oxide films and the oxide semiconductor film which have the microcrystalline structure each have a mixed phase structure where crystal parts (each of which is greater than or equal to 1 nm and less than 10 nm in size) are distributed in an amorphous phase, for example.

The oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 may have an amorphous structure, for example. The oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 which have the amorphous structure each have disordered atomic arrangement and no crystalline component, for example. Alternatively, an oxide film having an amorphous structure has, for example, an absolutely amorphous structure and no crystal part.

The oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 may each be a mixed film including regions having two or more of the following structures: a CAAC-OS, a microcrystalline structure, and an amorphous structure. The mixed film, for example, has a single-layer structure including a region having an amorphous structure, a region having a microcrystalline structure, and a region of a CAAC-OS. Alternatively, the mixed film may have a stacked-layer structure including a region having an amorphous structure, a region having a microcrystalline structure, and a region of a CAAC-OS, for example.

Note that the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 may have a single-crystal structure, for example.

The oxide insulating layers 121 and 123 function as barrier films that prevent constituent elements of the insulating layer 110 and the gate insulating layer 150 from entering the oxide semiconductor layer 122 and forming an impurity state.

For example, in the case of using a silicon-containing insulating film as the insulating layer 110 or the gate insulating layer 150, silicon in the gate insulating layer 150 or carbon which might be contained in the insulating layer 110 or the gate insulating layer 150 enters the oxide insulating layer 121 or 123 to a depth of several nanometers from the interface in some cases. An impurity, such as silicon or carbon, entering the oxide semiconductor layer 122 forms an impurity state. The impurity state serves as a donor to generate an electron; thus, the oxide semiconductor layer 122 might become n-type.

However, when each thickness of the oxide insulating layers 121 and 123 is larger than several nanometers, the impurity such as silicon or carbon does not reach the oxide semiconductor layer 122, so that the influence of impurity states is reduced.

Thus, providing the oxide insulating layers 121 and 123 makes it possible to reduce variations in electrical characteristics of the transistor, such as threshold voltage.

Accordingly, with the stacked-layer structure including the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123, a channel can be formed in the oxide semiconductor layer 122; thus, the transistor can have a high field-effect mobility and stable electrical characteristics.

Note that the oxide semiconductor layer does not necessarily have a three-layer structure including the oxide insulating layers and can have a single layer, two layers, four layers, or five or more layers. In the case of a single layer, a layer corresponding to the oxide semiconductor layer 122, which is described in this embodiment, can be used.

<Band Diagram>

Here, a band diagram of the transistor of one embodiment of the present invention will be described with reference to FIGS. 3A and 3B. For easy understanding, the band diagram in FIG. 3B is illustrated with the energy levels (Ec) at the conduction band minimum and the energy levels (Ev) at the valence band maximum of the insulating layer 110, the oxide insulating layer 121, the oxide semiconductor layer 122, the oxide insulating layer 123, and the gate insulating layer 150.

As illustrated in FIGS. 3A and 3B, the energy level at the conduction band minimum changes continuously within the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123. This can be understood also from the fact that the constituent elements are common among the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 and oxygen is easily diffused among them. Thus, the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 have a continuous physical property although they are a stack of films having different compositions.

Oxide semiconductor films, which contain the same main components and are stacked, are not simply stacked but formed to have continuous junction (here, particularly a U-shaped (U shape) well structure where the energy level at the conduction band minimum is continuously changed between the layers). In other words, a stacked-layer structure is formed such that there exist no impurities which form a defect state such as a trap center or a recombination center at each interface. If impurities are mixed between the stacked layers in the multilayer film, the continuity of the energy band is lost and carriers disappear by a trap or recombination at the interface.

Although Ec of the oxide insulating layer 121 and that of the oxide insulating layer 123 are equal to each other, they may be different.

As illustrated in FIG. 3B, the oxide semiconductor layer 122 serves as a well and a channel of the transistor 10 is formed in the oxide semiconductor layer 122. Note that a channel having a U-shaped well structure in which the energy level at the conduction band minimum continuously changes like the one formed in the oxide semiconductor layer 122, can also be referred to as a buried channel.

Note that trap states due to impurities or defects can be formed in the vicinity of the interface between an insulating film such as a silicon oxide film and each of the oxide insulating layers 121 and 123. The oxide semiconductor layer 122 can be distanced away from the trap states owing to existence of the oxide insulating layers 121 and 123. However, when the energy difference between Ec of the oxide insulating layer 121 or 123 and Ec of the oxide semiconductor layer 122 is small, an electron in the oxide semiconductor layer 122 might reach the trap state by passing over the energy difference. When electrons to be negative charge are captured by the trap states, a negative fixed charge is generated at the interface with the insulating film, whereby the threshold voltage of the transistor is shifted in the positive direction. In addition, a trap is not fixed and electrical characteristics can be changed in a long-time preservation test of the transistor.

Thus, to reduce a change in the threshold voltage of the transistor, the energy difference between the Ec of each of the oxide insulating layers 121 and 123 and the Ec of the oxide semiconductor layer 122 is necessary. The energy difference is preferably greater than or equal to 0.1 eV and further preferably greater than or equal to 0.2 eV.

The oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123 preferably include a crystal part. In particular, when a crystal in which c-axes are aligned is used, the transistor can have stable electrical characteristics.

In the band diagram illustrated in FIG. 3B, an In—Ga oxide (e.g., an In—Ga oxide with an atomic ratio of In:Ga=7:93), gallium oxide, or the like may be provided between the oxide semiconductor layer 122 and the gate insulating layer 150 without providing the oxide insulating layer 123. Alternatively, an In—Ga oxide, gallium oxide, or the like may be provided between the oxide insulating layer 123 and the gate insulating layer 150.

As the oxide semiconductor layer 122, an oxide having an electron affinity higher than those of the oxide insulating layers 121 and 123 is used. The oxide which can be used for the oxide semiconductor layer 122 has, for example, an electron affinity higher than that of each of the oxide insulating layers 121 and 123 by 0.07 eV or higher and 1.3 eV or lower, preferably 0.1 eV or higher and 0.7 eV or lower, and further preferably 0.2 eV or higher and 0.4 eV or lower.

Since the transistor described in this embodiment includes the oxide insulating layers 121 and 123 that each include one or more kinds of metal elements included in the oxide semiconductor layer 122, an interface state is less likely to be formed at the interface between the oxide insulating layer 121 and the oxide semiconductor layer 122 and the interface between the oxide insulating layer 123 and the oxide semiconductor layer 122. Thus, providing the oxide insulating layers 121 and 123 makes it possible to reduce variations or changes in electrical characteristics of the transistor, such as threshold voltage.

<Gate Insulating Layer 150>

The gate insulating layer 150 can contain oxygen (O), nitrogen (N), fluorine (F), aluminum (Al), magnesium (Mg), silicon (Si), gallium (Ga), germanium (Ge), yttrium (Y), zirconium (Zr), lanthanum (La), neodymium (Nd), hafnium (HO, tantalum (Ta), titanium (Ti), or the like. For example, the gate insulating layer 150 can contain one or more of aluminum oxide (AlO_(x)), magnesium oxide (MgO_(x)), silicon oxide (SiO_(x)), silicon oxynitride (SiO_(x)N_(y)), silicon nitride oxide (SiN_(x)O₃), silicon nitride (SiN_(x)), gallium oxide (GaO_(x)), germanium oxide (GeO_(x)), yttrium oxide (YO_(x)), zirconium oxide (ZrO_(x)), lanthanum oxide (LaO_(x)), neodymium oxide (NdO_(x)), hafnium oxide (HfO_(x)), and tantalum oxide (TaO_(x)). The gate insulating layer 150 may be a stack of any of the above materials. The gate insulating layer 150 may contain lanthanum (La), nitrogen, zirconium (Zr), or the like as an impurity.

The gate insulating layer 150 preferably contains a large amount of oxygen. Oxygen contained in the gate insulating layer 150 reaches the oxide semiconductor layer 122 through the oxide insulating layer 123 by heat treatment. Accordingly, oxygen vacancies (Vo) in the oxide semiconductor layer 122 can be reduced.

An example of a stacked-layer structure of the gate insulating layer 150 will be described. The gate insulating layer 150 contains, for example, oxygen, nitrogen, silicon, or hafnium. Specifically, the gate insulating layer 150 preferably contains hafnium oxide, and silicon oxide or silicon oxynitride.

Hafnium oxide has higher dielectric constant than silicon oxide and silicon oxynitride. Therefore, the gate insulating layer 150 using hafnium oxide can have larger thickness than the gate insulating layer 150 using silicon oxide, so that leakage current due to tunnel current can be reduced. That is, it is possible to provide a transistor with a low off-state current. Moreover, hafnium oxide with a crystalline structure has higher dielectric constant than hafnium oxide with an amorphous structure. Therefore, it is preferable to use hafnium oxide with a crystalline structure in order to provide a transistor with a low off-state current. Examples of the crystalline structure include a monoclinic crystal structure and a cubic crystal structure. Note that one embodiment of the present invention is not limited to the above examples.

A surface over which the hafnium oxide with a crystalline structure is formed might have interface states due to defects. The interface state serves as a trap center in some cases. Therefore, when hafnium oxide is provided near a channel region of a transistor, the electrical characteristics of the transistor might deteriorate because of the interface state. In order to reduce the adverse effect of the interface state, in some cases, it is preferable to separate the channel region of the transistor and the hafnium oxide from each other by providing another film therebetween. The film has a buffer function. The film having a buffer function may be included in the gate insulating layer 150 or included in an oxide semiconductor film. That is, the film having a buffer function can be formed using silicon oxide, silicon oxynitride, an oxide semiconductor, or the like. Note that the film having a buffer function is formed using, for example, a semiconductor or an insulator having a larger energy gap than a semiconductor to be the channel region. Alternatively, the film having a buffer function is formed using, for example, a semiconductor or an insulator having lower electron affinity than a semiconductor to be the channel region. Further alternatively, the film having a buffer function is formed using, for example, a semiconductor or an insulator having higher ionization energy than a semiconductor to be the channel region.

In some cases, the threshold voltage of a transistor can be controlled by trapping an electric charge in an interface state (trap center) at the surface over which the hafnium oxide with a crystalline structure is formed. In order to make the electric charge exist stably, for example, an insulator having a larger energy gap than hafnium oxide may be provided between the channel region and the hafnium oxide. Alternatively, a semiconductor or an insulator having lower electron affinity than hafnium oxide may be provided. The film having a buffer function may be formed using a semiconductor or an insulator having higher ionization energy than hafnium oxide. With the use of such an insulator, an electric charge trapped in the interface state is less likely to be released; accordingly, the electric charge can be held for a long period of time.

Examples of such an insulator include silicon oxide and silicon oxynitride. In order to make the interface state in the gate insulating layer 150 trap an electric charge, an electron is transferred from the oxide semiconductor layer 122 toward the gate electrode layer 160. As a specific example, the potential of the gate electrode layer 160 is kept higher than the potential of the source or the drain at high temperatures (e.g., a temperature higher than or equal to 125° C. and lower than or equal to 450° C., typically higher than or equal to 150° C. and lower than or equal to 300° C.) for one second or longer, typically for one minute or longer.

The threshold voltage of a transistor in which a predetermined amount of electrons are trapped in interface states in the gate insulating layer 150 or the like shifts in the positive direction. The amount of electrons to be trapped (the amount of change in threshold voltage) can be controlled by adjusting a voltage of the gate electrode layer 160 or time in which the voltage is applied. Note that a location in which an electric charge is trapped is not necessarily limited to the inside of the gate insulating layer 150 as long as an electric charge can be trapped therein. A stacked film having a similar structure may be used as a different insulating layer.

<Gate Electrode Layer 160>

The gate electrode layer 160 can be formed using aluminum (Al), titanium (Ti), chromium (Cr), cobalt (Co), nickel (Ni), copper (Cu), yttrium (Y), zirconium (Zr), molybdenum (Mo), ruthenium (Ru), silver (Ag), tantalum (Ta), tungsten (W), or silicon (Si), for example. The gate electrode layer 160 may have a stacked-layer structure. When a stacked-layer structure is employed, the above materials may be combined with a material containing nitrogen, such as a nitride of any of the above materials.

<Sidewall Insulating Layer 176>

The sidewall insulating layer 176 can be formed using a material similar to that of the gate insulating layer 150.

<Insulating Layer 180>

The insulating layer 180 can be formed using a material similar to that of the gate insulating layer 150.

The insulating layer 180 may be a stack. The insulating layer 180 preferably contains oxygen more than that in the stoichiometric composition. Oxygen released from the insulating layer 180 can be diffused into the channel formation region in the oxide semiconductor layer 122 through the gate insulating layer 150, so that oxygen vacancies formed in the channel formation region can be filled with the oxygen. In this manner, stable electrical characteristics of the transistor can be achieved.

<Conductive Layer 190>

The conductive layer 190 can be formed using a material similar to that of the gate electrode layer 160.

<Conductive Layer 195>

The conductive layer 195 can be formed using a material similar to that of the gate electrode layer 160.

<Manufacturing Method of Transistor>

Next, a manufacturing method of a semiconductor device of this embodiment will be described with reference to FIGS. 4A to 4D, FIGS. 5A and 5B, FIGS. 6A to 6C, FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS. 9A to 9C, FIGS. 10A to 10C, FIGS. 11A to 11C, FIGS. 12A to 12C, FIGS. 13A to 13C, FIGS. 14A to 14C, FIGS. 15A to 15C, FIGS. 16A to 16C, FIGS. 17A to 17C, and FIGS. 18A to 18C. Note that the same parts as those in the above transistor structure are not described here. The direction of A1-A2 and that of A3-A4 in FIGS. 6A to 6C, FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS. 9A to 9C, FIGS. 10A to 10C, FIGS. 11A to 11C, FIGS. 12A to 12C, FIGS. 13A to 13C, FIGS. 14A to 14C, FIGS. 15A to 15C, FIGS. 16A to 16C, FIGS. 17A to 17C, and FIGS. 18A to 18C are respectively referred to as a channel length direction in FIGS. 1A and 1B and a channel width direction in FIGS. 1A and 1C in some cases.

In this embodiment, the layers included in the transistor (i.e., the insulating layer, the oxide semiconductor layer, the conductive layer, and the like) can be deposited by any of a sputtering method, a chemical vapor deposition (CVD) method, a vacuum evaporation method, and a pulsed laser deposition (PLD) method. Alternatively, a coating method or a printing method can be used. Although the sputtering method and a plasma-CVD method are typical examples of the film formation method, a thermal CVD method may be used. As the thermal CVD method, a metal organic chemical vapor deposition (MOCVD) method or an atomic layer deposition (ALD) method may be used, for example. As the sputtering method, a combination of a long throw sputtering method and a collimated sputtering method is employed, whereby the embeddability can be improved.

<Thermal CVD Method>

A thermal CVD method has an advantage that no defect due to plasma damage is generated because it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a manner that a source gas and an oxidizer are supplied to a chamber at a time, the pressure in the chamber is set to an atmospheric pressure or a reduced pressure, and reaction is caused in the vicinity of the substrate or over the substrate.

The variety of films such as the metal film, the semiconductor film, and the inorganic insulating film which have been disclosed in the embodiment can be deposited by a thermal CVD method such as a MOCVD method or an ALD method. For example, in the case where an In—Ga—Zn—O film is formed, trimethylindium, trimethylgallium, and dimethylzinc can be used. Note that the chemical formula of trimethylindium is In(CH₃)₃. The chemical formula of trimethylgallium is Ga(CH₃)₃. The chemical formula of dimethylzinc is Zn(CH₃)₂. Without limitation to the above combination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be used instead of trimethylgallium, and diethylzinc (chemical formula: Zn(C₂H₅)₂) can be used instead of dimethylzinc.

<ALD Method>

In a conventional deposition apparatus utilizing a CVD method, one or more kinds of source gases (precursors) for reaction are supplied to a chamber at the same time at the time of deposition. In a deposition apparatus utilizing an ALD method, precursors for reaction are sequentially introduced into a chamber, and then the sequence of the gas introduction is repeated. For example, two or more kinds of precursors are sequentially supplied to the chamber by switching respective switching valves (also referred to as high-speed valves). For example, a first precursor is introduced, an inert gas (e.g., argon or nitrogen), or the like is introduced after the introduction of the first precursor so that the plural kinds of precursors are not mixed, and then a second precursor is introduced. Alternatively, the first precursor may be exhausted by vacuum evacuation instead of the introduction of the inert gas, and then the second precursor may be introduced.

FIGS. 4A to 4D illustrate a deposition process by an ALD method. First precursors 601 are adsorbed onto a substrate surface (see FIG. 4A), whereby a first monolayer is formed (see FIG. 4B). At this time, metal atoms and the like included in the precursors can be bonded to hydroxyl groups that exist at the substrate surface. The metal atoms may be bonded to alkyl groups such as methyl groups or ethyl groups. The first monolayer reacts with second precursors 602 introduced after the first precursors 601 are evacuated (see FIG. 4C), whereby a second monolayer is stacked over the first monolayer. Thus, a thin film is formed (see FIG. 4D). For example, in the case where an oxidizer is included in the second precursors, the oxidizer chemically reacts with metal atoms included in the first precursors or an alkyl group bonded to metal atoms, whereby an oxide film can be formed.

An ALD method is a deposition method based on a surface chemical reaction, by which precursors are adsorbed onto a surface and adsorbing is stopped by a self-terminating mechanism, whereby a layer is deposited. For example, precursors such as trimethylaluminum react with hydroxyl groups (OH groups) that exist at the surface. At this time, only a surface reaction due to heat occurs; therefore, the precursors come into contact with the surface and metal atoms or the like in the precursors can be adsorbed onto the surface through thermal energy. The precursors have characteristics of, for example, having a high vapor pressure, being thermally stable before being deposited and not dissolving, and being chemically adsorbed onto a substrate at a high speed. Since the precursors are introduced in a state of a gas, when the precursors, which are alternately introduced, have enough time to be diffused, a film can be deposited with good coverage even onto a region having unevenness with a high aspect ratio.

In an ALD method, the sequence of the gas introduction is repeated a plurality of times until a desired thickness is obtained, whereby a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of the gas introduction; therefore, an ALD method makes it possible to accurately adjust a thickness. The deposition rate can be increased and the impurity concentration in the film can be further reduced by improving the evacuation capability.

ALD methods include an ALD method using heating (thermal ALD method) and an ALD method using plasma (plasma ALD method). In the thermal ALD method, precursors react using thermal energy, and in the plasma ALD method, precursors react in a state of a radical.

When a film is deposited by an ALD method, an extremely thin film can be deposited with high accuracy. In addition, even the coverage of an uneven surface with the film can be improved by forming a film with an ALD method.

<Plasma ALD>

Alternatively, when the plasma ALD method is employed, the film can be deposited at a lower temperature than when the thermal ALD method is employed. With the plasma ALD method, for example, the film can be deposited without decreasing the deposition rate even at 100° C. or lower. Moreover, in the plasma ALD method, nitrogen radicals can be formed by plasma; thus, a nitride film as well as an oxide film can be formed.

In addition, oxidizability of an oxidizer can be enhanced by the plasma ALD method. Thus, precursors remaining in a plasma ALD film or organic components released from precursors can be reduced. In addition, carbon, chlorine, hydrogen, and the like in the film can be reduced. Therefore, a film with low impurity concentration can be formed.

In the case of using the plasma ALD method, radical species are generated and plasma can be generated from a place apart from the substrate like inductively coupled plasma (ICP) or the like, so that plasma damage to the substrate or a film on which the protective film is formed can be inhibited.

As described above, with the plasma ALD method, the film can be deposited in the state where the process temperature can be lowered and the coverage of the surface can be increased as compared with other deposition methods. Thus, entry of water and hydrogen from the outside can be inhibited, leading to an improvement of the reliability of characteristics of the transistor.

<ALD Apparatus>

FIG. 5A illustrates an example of a deposition apparatus utilizing an ALD method. The deposition apparatus utilizing an ALD method includes a deposition chamber (chamber 1701), source material supply portions 1711 a and 1711 b, high-speed valves 1712 a and 1712 b which are flow rate controllers, source material introduction ports 1713 a and 1713 b, a source material exhaust port 1714, and an evacuation unit 1715. The source material introduction ports 1713 a and 1713 b provided in the chamber 1701 are connected to the source material supply portions 1711 a and 1711 b, respectively, through supply tubes and valves. The source material exhaust port 1714 is connected to the evacuation unit 1715 through an exhaust tube, a valve, and a pressure controller.

A substrate holder 1716 with a heater is provided in the chamber, and a substrate 1700 over which a film is deposited is provided over the substrate holder.

In the source material supply portions 1711 a and 1711 b, a precursor is formed from a solid source material or a liquid source material by using a vaporizer, a heating unit, or the like. Alternatively, the source material supply portions 1711 a and 1711 b may supply a precursor in a gas state.

Although two source material supply portions 1711 a and 1711 b are provided in this example, the number of source material supply portions is not limited thereto, and three or more source material supply portions may be provided. The high-speed valves 1712 c and 1712 d can be accurately controlled by time, and supply one of a precursor and an inert gas. The high-speed valves 1712 c and 1712 d are flow rate controllers for a precursor, and can also be referred to as flow rate controllers for an inert gas.

In the deposition apparatus illustrated in FIG. 5A, a thin film is formed over a surface of the substrate 1700 in the following manner: the substrate 1700 is transferred to be put on the substrate holder 1716; the chamber 1701 is sealed; the substrate 1700 is heated to a desired temperature (e.g., higher than or equal to 100° C. or higher than or equal to 150° C.) by heating the substrate holder 1716 with a heater; and supply of a precursor, evacuation with the evacuation unit 1715, supply of an inert gas, and evacuation with the evacuation unit 1715 are repeated.

In the deposition apparatus illustrated in FIG. 5A, an insulating layer formed using an oxide (including a composite oxide) containing one or more elements selected from hafnium, aluminum, tantalum, zirconium, and the like can be deposited by selecting as appropriate a source material (e.g., a volatile organometallic compound) used for the source material supply portions 1711 a and 1711 b. Specifically, it is possible to deposit an insulating layer including hafnium oxide, an insulating layer including aluminum oxide, an insulating layer including hafnium silicate, or an insulating layer including aluminum silicate. Alternatively, a thin film, e.g., a metal layer such as a tungsten layer or a titanium layer, or a nitride layer such as a titanium nitride layer can be deposited by selecting as appropriate a source material (e.g., a volatile organometallic compound) used for the source material supply portions 1711 a and 1711 b.

For example, in the case where a hafnium oxide layer is formed with a deposition apparatus using an ALD method, two kinds of gases, i.e., ozone (O₃) as an oxidizer and a precursor which is obtained by vaporizing liquid containing a solvent and a hafnium precursor compound (hafnium alkoxide or hafnium amide such as tetrakis(dimethylamide)hafnium (TDMAH)), are used. In this case, the first precursor supplied from the source material supply portion 1711 a is TDMAH, and the second precursor supplied from the source material supply portion 1711 b is ozone. Note that the chemical formula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples of another material include tetrakis(ethylmethylamide)hafnium. Note that nitrogen has a function of eliminating charge trap states. Therefore, when the precursor contains nitrogen, a hafnium oxide film having low density of charge trap states can be deposited.

For example, in the case where an aluminum oxide layer is formed with a deposition apparatus utilizing an ALD method, two kinds of gases, i.e., H₂O as an oxidizer and a precursor which is obtained by vaporizing liquid containing a solvent and an aluminum precursor compound (e.g., trimethylaluminum (TMA)), are used. In this case, the first precursor supplied from the source material supply portion 1711 a is TMA, and the second precursor supplied from the source material supply portion 1711 b is H₂O. Note that the chemical formula of trimethylaluminum is Al(CH₃)₃. Examples of another material liquid include tris(dimethylamide)aluminum, triisobutylaluminum, and aluminum tris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed with a deposition apparatus using an ALD method, hexachlorodisilane is adsorbed on a surface where a film is to be formed, chlorine contained in the adsorbate is removed, and radicals of an oxidizing gas (e.g., O₃ or dinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is deposited using a deposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas are sequentially introduced plural times to form an initial tungsten film, and then a WF₆ gas and an H₂ gas are sequentially introduced plural times to form a tungsten film. Note that an SiH₄ gas may be used instead of a B₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., an In—Ga—Zn—O film is deposited using a deposition apparatus employing ALD, an In(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times to form an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas are sequentially introduced plural times to form a ZnO layer. Note that the order of these layers is not limited to this example.

A mixed compound layer such as an In—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by mixing these gases. Note that although an H₂O gas which is obtained by bubbling with an inert gas such as Ar may be used instead of an O₃ gas, it is preferable to use an O₃ gas, which does not contain H. Instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂ gas may be used.

<Multi-Chamber Manufacturing Apparatus>

FIG. 5B illustrates an example of a multi-chamber manufacturing apparatus including at least one deposition apparatus illustrated in FIG. 5A.

In the manufacturing apparatus illustrated in FIG. 5B, a stack of films can be successively deposited without exposure to the air, and entry of impurities is prevented and throughput is improved.

The manufacturing apparatus illustrated in FIG. 5B includes at least a load chamber 1702, a transfer chamber 1720, a pretreatment chamber 1703, a chamber 1701 which is a deposition chamber, and an unload chamber 1706. Note that in order to prevent attachment of moisture, the chambers of the manufacturing apparatus (including the load chamber, the treatment chamber, the transfer chamber, the deposition chamber, the unload chamber, and the like) are preferably filled with an inert gas (such as a nitrogen gas) whose dew point is controlled and further preferably maintain reduced pressure.

Chambers 1704 and 1705 may be deposition apparatuses utilizing an ALD method like the chamber 1701, deposition apparatuses utilizing a plasma CVD method, deposition apparatuses utilizing a sputtering method, or deposition apparatuses utilizing a MOCVD method.

For example, an example in which a stack of films is formed under a condition that the chamber 1704 is a deposition apparatus utilizing a plasma CVD method and the chamber 1705 is a deposition apparatus utilizing an MOCVD method is described below.

Although FIG. 5B illustrates an example in which a top view of the transfer chamber 1720 is a hexagon, a manufacturing apparatus in which the top surface shape is set to a polygon having more than six corners and more chambers are connected depending on the number of layers of a stack may be used. The top surface shape of the substrate is rectangular in FIG. 5B; however, there is no particular limitation on the top surface shape of the substrate. Although FIG. 5B illustrates an example of the single wafer type, a batch-type deposition apparatus in which films are deposited on a plurality of substrates at a time may be used.

<Formation of Insulating Layer 110>

First, the insulating layer 110 is deposited over the substrate 100. The insulating layer 110 can be formed by a plasma CVD method, a thermal CVD method (an MOCVD method, or an ALD method), a sputtering method, or the like using an oxide insulating film of aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon oxyfluoride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, or the like, a nitride insulating film of silicon nitride, silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or the like, or a mixed material of any of these. Alternatively, a stack of the above materials may be used.

The use of a material that contains no hydrogen or a material that contains hydrogen at 1% or lower for the insulating layer 110 can prevent oxygen vacancies from being generated in the oxide semiconductor, leading to stable operation of the transistor.

As the insulating layer 110, for example, a 100-nm-thick silicon oxynitride film can be formed by a plasma CVD method.

Next, first heat treatment may be performed to release water, hydrogen, or the like contained in the insulating layer 110. As a result, the concentration of water, hydrogen, or the like contained in the insulating layer 110 can be reduced. The heat treatment can reduce the amount of water, hydrogen, or the like diffused into a first oxide insulating film that is to be formed later.

The temperature of the first heat treatment is higher than or equal to 250° C. and lower than the strain point of the substrate, preferably higher than or equal to 300° C. and lower than or equal to 650° C., further preferably higher than or equal to 350° C. and lower than or equal to 550° C.

The first heat treatment is preferably performed under an inert gas atmosphere containing nitrogen or a rare gas such as helium, neon, argon, xenon, or krypton. Further, after heat treatment performed in an inert gas atmosphere, heat treatment may be additionally performed in an oxygen atmosphere or a dry air atmosphere (air whose dew point is lower than or equal to −80° C., preferably lower than or equal to −100° C. and further preferably lower than or equal to −120° C.). The treatment may be performed under reduced pressure. Note that it is preferable that hydrogen, water, and the like not be contained in an inert gas and oxygen, like the dry air, and the dew point is preferably lower than or equal to −80° C. and further preferably lower than or equal to −100° C. The treatment time is preferably 30 seconds to 24 hours.

In the first heat treatment, instead of an electric furnace, any device for heating an object by heat conduction or heat radiation from a heating element, such as a resistance heating element, may be used. For example, an RTA (rapid thermal annealing) apparatus, such as a GRTA (gas rapid thermal annealing) apparatus or an LRTA (lamp rapid thermal annealing) apparatus, can be used. The LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp, such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. The GRTA apparatus is an apparatus for the heat treatment using a high-temperature gas. As the high-temperature gas, an inert gas, such as nitrogen or a rare gas like argon, is used.

<Formation of First Oxide Insulating Film and Oxide Semiconductor Film>

Then, the first oxide insulating film to be the oxide insulating layer 121 later and the oxide semiconductor film to be the oxide semiconductor layer 122 later are deposited over the insulating layer 110. The first oxide insulating film and the oxide semiconductor film can be formed by a sputtering method, an MOCVD method, a PLD method, or the like, and especially, a sputtering method is preferable. As a sputtering method, an RF sputtering method, a DC sputtering method, an AC sputtering method, or the like can be used. In addition, a facing-target-type sputtering method (also referred to as a counter-electrode-type sputtering method, a gas phase sputtering method, and a vapor deposition sputtering (VDSP) method) is used, whereby plasma damage at the deposition can be reduced.

When the oxide semiconductor film is formed by a sputtering method, for example, it is preferable that each chamber of the sputtering apparatus be able to be evacuated to a high vacuum (approximately 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) by an adsorption vacuum pump such as a cryopump, and that the chamber be able to heat a substrate over which a film is to be deposited to 100° C. or higher, preferably 400° C. or higher, so that water and the like acting as impurities in the oxide semiconductor layer 122 can be removed as much as possible. Alternatively, a combination of a turbo molecular pump and a cold trap is preferably used to prevent back-flow of a gas containing a carbon component, moisture, or the like from an exhaust system into the chamber. Alternatively, a combination of a turbo molecular pump and a cryopump may be used as an exhaust system.

Not only high vacuum evacuation in a chamber but also high purity of a sputtering gas is desirable to obtain a highly purified intrinsic oxide semiconductor layer. As an oxygen gas or an argon gas used as a sputtering gas, a highly purified gas having a dew point of −40° C. or lower, preferably −80° C. or lower and further preferably −100° C. or lower, is used, whereby moisture or the like can be prevented from entering an oxide semiconductor film as much as possible.

As a sputtering gas, a rare gas (typically argon), oxygen, or a mixed gas of a rare gas and oxygen can be used as appropriate.

Note that for example, in the case where the oxide semiconductor film is deposited by a sputtering method at a substrate temperature higher than or equal to 20° C. and lower than or equal to 750° C., preferably higher than or equal to 150° C. and lower than or equal to 450° C. and further preferably higher than or equal to 200° C. and lower than or equal to 420° C., the oxide semiconductor film can be a CAAC-OS film.

The material for the first oxide insulating film is desirably selected so that the first oxide insulating film can have a lower electron affinity than the oxide semiconductor film.

For example, when a sputtering method is used to deposit the first oxide insulating film and the oxide semiconductor film, the first oxide insulating film and the oxide semiconductor film can be successively deposited without being exposed to the air with the use of a multi-chamber sputtering apparatus. In that case, entry of unnecessary impurities and the like into the interface between the first oxide insulating film and the oxide semiconductor film can be prevented and interface states can be reduced accordingly. Thus, the electrical characteristics of a transistor can be stabilized, particularly in a reliability test.

If the insulating layer 110 is damaged, the oxide semiconductor layer 122, which is a main conduction path, can keep a distance from the damaged part owing to the existence of the oxide insulating layer 121. Thus, the electrical characteristics of a transistor can be stabilized, particularly in a reliability test.

For example, as the first oxide insulating film, a 20-nm-thick oxide insulating film which is deposited by a sputtering method using a target having an atomic ratio of In:Ga:Zn=1:3:4 can be used. In addition, as the oxide semiconductor film, a 15-nm-thick oxide semiconductor film which is deposited by a sputtering method using a target having an atomic ratio of In:Ga:Zn=1:1:1 can be used.

Note that the amount of oxygen vacancies in the oxide semiconductor film can be reduced by performing second heat treatment after the first oxide insulating film and the oxide semiconductor film are deposited.

The temperature of the second heat treatment is preferably higher than or equal to 250° C. and lower than the strain point of the substrate, further preferably higher than or equal to 300° C. and lower than or equal to 650° C., and still further preferably higher than or equal to 350° C. and lower than or equal to 550° C.

The second heat treatment can be performed in a manner similar to the first heat treatment.

Note that the second heat treatment may be performed after etching for forming the oxide insulating layer 121 and the oxide semiconductor layer 122 described later.

For example, after heat treatment is performed at 450° C. in a nitrogen atmosphere for one hour, heat treatment is performed at 450° C. in an oxygen atmosphere for one hour.

Through the above-described steps, oxygen vacancies and impurities such as hydrogen and water in the oxide semiconductor film can be reduced. The oxide semiconductor film can have low density of localized states.

Note that high-density plasma irradiation using oxygen can exert an effect equivalent to that exerted by heat treatment. Irradiation time is longer than or equal to 1 minute and shorter than or equal to 3 hours, preferably longer than or equal to 3 minutes and shorter than or equal to 2 hours, and further preferably longer than or equal to 5 minutes and shorter than or equal to 1 hour.

<Formation of First Conductive Film>

Next, a first conductive film used as a hard mask is formed over the oxide semiconductor film. The first conductive film can be formed by a sputtering method, a chemical vapor deposition (CVD) method such as a metal organic chemical vapor deposition (MOCVD) method, a metal chemical vapor deposition method, an atomic layer deposition (ALD) method, or a plasma-enhanced chemical vapor deposition (PECVD) method, an evaporation method, a pulsed laser deposition (PLD) method, or the like.

The first conductive film is preferably, for example, a conductive film having a single-layer structure or a stacked-layer structure and containing a material selected from copper (Cu), tungsten (W), molybdenum (Mo), gold (Au), aluminum (Al), manganese (Mn), titanium (Ti), tantalum (Ta), nickel (Ni), chromium (Cr), lead (Pb), tin (Sn), iron (Fe), cobalt (Co), ruthenium (Ru), platinum (Pt), iridium (Ir), and strontium (Sr), an alloy of such a material, or a compound containing such a material as its main component.

As the first conductive film, for example, a tungsten film having a thickness of 20 nm to 100 nm can be formed by a sputtering method.

Although the first conductive film is formed as a hard mask in this embodiment, one embodiment of the present invention is not limited thereto, and an insulating film may be formed instead.

<Formation of Oxide Insulating Layer 121 and Oxide Semiconductor Layer 122>

Next, a resist mask is formed over the first conductive film by a lithography process. When the resist mask is formed, the first conductive film is selectively etched using the resist mask to form a hard mask. After the resist over the hard mask is removed, the oxide semiconductor film and the first oxide insulating film are selectively etched, whereby the island-shaped oxide semiconductor layer 122 and oxide insulating layer 121 are formed (see FIGS. 6A to 6C). Dry etching can be used here.

For example, the first oxide insulating film and the oxide semiconductor film are selectively etched using a resist mask and a hard mask with a methane gas and an argon gas as an etching gas, whereby the oxide insulating layer 121 and the oxide semiconductor layer 122 can be formed. After the oxide insulating layer 121 and the oxide semiconductor layer 122 are formed, the first conductive film is removed.

Note that the use of the first conductive film as a hard mask for etching of the oxide semiconductor layer can reduce edge roughness of the oxide semiconductor layer after the etching as compared with the case of using a resist mask.

<Formation of Oxide Insulating Layer 123>

Next, an oxide insulating layer 123 a to be the oxide insulating layer 123 is deposited over the oxide semiconductor layer 122 and the insulating layer 110 (see FIGS. 7A to 7C). The oxide insulating layer 123 a can be deposited in a manner similar to that of the oxide semiconductor film and the first oxide insulating film. The materials can be selected such that the electron affinity of the oxide insulating layer 123 a is smaller than that of the oxide semiconductor film. Note that the oxide insulating layer 123 a may be processed using a resist mask or the gate electrode layer 160 as a mask.

For example, as the oxide insulating layer 123 a, a 5-nm-thick oxide semiconductor film which is deposited by a sputtering method using a target having an atomic ratio of In:Ga:Zn=1:3:2 can be used.

<Formation of Insulating Film 150 a>

Next, an insulating film 150 a to be the gate insulating layer 150 is formed over the oxide insulating layer 123 a. The insulating film 150 a may be a stack containing any of these materials. The insulating film 150 a can be formed by a sputtering method, a CVD method (e.g., a plasma CVD method, an MOCVD method, or an ALD method), an MBE method, or the like. The insulating film 150 a can be formed by a method similar to that of the insulating layer 110 as appropriate.

For example, as the insulating film 150 a, silicon oxynitride can be deposited to a thickness of 10 nm by a plasma CVD method.

<Formation of Conductive Film 160 a>

Next, a conductive film 160 a to be the gate electrode layer 160 is deposited over the insulating film 150 a (see FIGS. 8A to 8C). The conductive film 160 a can be formed by a sputtering method, a CVD method (e.g., a plasma CVD method, an MOCVD method, or an ALD method), an MBE method, an evaporation method, a plating method, or the like. The conductive film 160 a may be formed using a conductive film containing nitrogen or a stack including the above conductive film and a conductive film containing nitrogen.

For example, a stack of 10-nm-thick titanium nitride deposited by an ALD method and 150-nm-thick tungsten deposited by a metal CVD method can be used as the conductive film 160 a.

<Formation of Gate Electrode Layer 160 and Gate Insulating Layer 150>

Then, a resist mask is formed over the conductive film 160 a through a lithography process. The conductive film 160 a is selectively etched using the resist mask, so that the gate electrode layer 160 is formed. After the resist over the gate electrode layer 160 is removed, the insulating film 150 a and the oxide insulating layer 123 a are selectively etched using the gate electrode layer 160 as a mask, so that the gate insulating layer 150 and the oxide insulating layer 123 are formed (see FIGS. 9A to 9C).

<Modification Example of Gate Electrode Layer 160 and Gate Insulating Layer 150>

Note that the formation method of the gate electrode layer 160 and the gate insulating layer 150 is not limited to the above method. For example, the gate electrode layer 160 and the gate insulating layer 150 may be formed in such a manner that a groove portion is provided and then an insulating film and a conductive film are embedded therein.

[Deposition of First Insulating Film]

First, a first insulating film to be an insulating layer 175 later is deposited over the oxide insulating layer 123. The first insulating film can be deposited using a material and a method similar to those of the insulating layer 110.

[Planarization of First Insulating Film]

Then, planarization treatment is performed on the first insulating film, so that an insulating layer 175 b is formed (see FIGS. 10A to 10C). The planarization treatment can be performed by a chemical mechanical polishing (CMP) method, a dry etching method, a reflow method, or the like. In the case where the CMP method is used, a film whose composition is different from that of the first insulating film is formed over the first insulating film, whereby the thickness of the insulating layer 175 b in the substrate surface after the CMP treatment can be uniform.

[Formation of Groove Portion]

Next, a resist mask is formed over the planarized insulating layer 175 b by a lithography process. Note that the lithography process may be performed after an organic film is applied to the insulating layer or after an organic film is applied to the resist mask. The organic film can contain propylene glycolmonomethyl ether, ethyl lactate, or the like. The use of the organic film leads to, in addition to an anti-reflection effect during light exposure, an improvement in adhesion between a resist and a film, an improvement in resolution, and the like. The organic film can be used in another process.

Note that in the case where a transistor having an extremely short channel length is formed, etching is performed using a resist mask that is processed by a method suitable for micropatterning, such as electron beam exposure, liquid immersion exposure, or extreme ultraviolet (EUV) exposure. Not that in the case of forming the resist mask by electron beam exposure, a positive resist mask is used, so that an exposed region can be minimized and throughput can be improved. In the above manner, a transistor having a channel length of 100 nm or less, further, 30 nm or less, still further, 20 nm or less can be formed. Alternatively, minute processing may be performed by an exposure technology which uses X-rays or the like.

With the use of the resist mask, groove processing is performed on the insulating layer 175 b by a dry etching method until the oxide insulating layer 123 is exposed. As a result of the processing, the insulating layer 175 and the groove portion 174 are formed.

Note that the groove portion 174 is preferably perpendicular to a substrate surface.

Note that the processing method of the groove portion 174 is not limited to the above method. For example, not only the resist mask but also a hard mask may be used, or a half-tone mask may be used in a lithography process to control the shape of the resist mask. Alternatively, the shape of the mask may be controlled by nanoimprint lithography or the like. The nanoimprint lithography can be used in another process.

Next, the insulating film 150 a to be the gate insulating layer 150 is formed over the oxide insulating layer 123 and the insulating layer 175.

Next, the conductive film 160 a to be the gate electrode layer 160 is deposited over the insulating film 150 a (see FIGS. 11A to 11C).

[Planarization Treatment]

Next, planarization treatment is performed (see FIGS. 12A to 12C). The planarization treatment can be preformed by a CMP method, a dry etching method, or the like. The planarization treatment may be terminated at the time when the insulating film 150 a is exposed, or may be terminated at the time when the insulating layer 175 is exposed. Accordingly, the gate electrode layer 160 and an insulating layer 150 b can be formed.

[Etch-Back Treatment of Insulating Layer 175]

Next, the insulating layer 175 and the insulating layer 150 b are subjected to etch-back treatment by a dry etching method so that the oxide insulating layer 123 is exposed. Furthermore, part of the oxide insulating layer 123 not overlapping with the gate electrode layer 160 is etched, whereby the gate insulating layer 150 can be formed (see FIGS. 13A to 13C). Note that the gate insulating layer 150 may remain on a side surface of the gate electrode layer.

With use of the above method, the gate electrode layer 160 and the gate insulating layer 150 illustrated in FIGS. 13A to 13C can be formed.

<Formation of Sidewall Insulating Layer 176>

Next, a second insulating film to be the sidewall insulating layer 176 is formed over the insulating layer 110, the oxide semiconductor layer 122, and the gate electrode layer 160. Etching treatment by a dry etching method is performed on the second insulating film, whereby the sidewall insulating layer 176 which includes a region that is in contact with side surfaces of the gate electrode layer 160 and the gate insulating layer 150 is formed (see FIGS. 13A to 13C).

<Formation of Low-Resistance Region 127>

Next, a conductive layer 166 is deposited over the insulating layer 110, the oxide semiconductor layer 122, the gate electrode layer 160, and the sidewall insulating layer 176. As the conductive layer 166, a metal material such as cobalt (Co), nickel (Ni), platinum (Pt), vanadium (V), chromium (Cr), and palladium (Pd) can be used. The metal elements are diffused into the oxide semiconductor layer 122 side and the oxide insulating layer 123 side by performing heat treatment after deposition.

For example, cobalt can be deposited to a thickness of 10 nm as the conductive layer 166 by a sputtering method.

Next, a conductive layer 168 is deposited over the conductive layer 166 (see FIGS. 14A to 14C). As the conductive layer 168, a metal material such as titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), rhodium (Rh), iridium (Ir), or cobalt (Co) is preferably used. In the case where heat treatment is performed after deposition of the metal, metal elements which are constituent elements of the oxide semiconductor layer 122 and the oxide insulating layer 123 can be diffused into the conductive layer 168 side.

For example, titanium can be deposited to a thickness of 20 nm as the conductive layer 168 by a sputtering method.

Next, third heat treatment is preferably performed. The temperature of the heat treatment is higher than or equal to 250° C. and lower than the strain point of the substrate, preferably higher than or equal to 400° C. and lower than or equal to 800° C., and further preferably higher than or equal to 450° C. and lower than or equal to 650° C.

The third heat treatment can be performed in a manner similar to that of the first heat treatment. Note that an RTA method is preferably used because the process time can be shortened.

The third heat treatment is preferably performed in an atmosphere without oxygen.

For example, the third heat treatment can be performed at 500° C. in an nitrogen atmosphere for 60 seconds by an RTA method.

Elements contained in the oxide semiconductor layer 122 and the conductive layer 166 are diffused by the third heat treatment (see FIGS. 15A to 15C).

FIG. 55 is an enlarged view of part of FIG. 15B. A constituent element 167 of the conductive layer 166 is diffused from the conductive layer 166 to the oxide semiconductor layer 122 by the third heat treatment. A constituent element 169 in the oxide semiconductor layer 122 is diffused from the oxide semiconductor layer 122 to the conductive layer 168 by the third heat treatment. Such diffusions in the two directions can lower the energy barrier in diffusion as compared with the case where any one of the conductive layer 166 and the conductive layer 168 exists alone. As a result, the diffusion of each element can be promoted and the diffusion rate thereof can be increased. Note that the above diffusion may involve the diffusion of a constituent element of the conductive layer 168.

As described above, when constituent elements of the oxide semiconductor layer 122, the conductive layer 166, and the conductive layer 168 are diffused from one layer to another layer, the low-resistance region 127 can be formed (see FIGS. 16A to 16C).

After the low-resistance region 127 is formed, the conductive layer 166 and the conductive layer 168 are removed (see FIGS. 17A to 17C). The conductive layers 166 and 168 may be removed by either a wet etching method or a dry etching method. For example, the conductive layer 166 and the conductive layer 168 can be removed by a mixture of ammonium water or a hydrogen peroxide solution and a mixture of hydrochloric acid and a hydrogen peroxide solution.

The low-resistance region 127 can include a region which is alloyed. In this case, the region which is alloyed can be left without being etched.

The region which is alloyed reaches the conductive layer 166 and the conductive layer 168 in some cases. For this reason, the distance between the top surface of the low-resistance region 127 and the top surface of the substrate 100 and the distance between a top surface of the oxide semiconductor layer 122 and the top surface of the substrate 100 are may be the same or different from each other.

The formed low-resistance region 127 can function as a source region and a drain region. The second region overlapping with the sidewall insulating layer 176 includes part of the low-resistance region 127. The third region not overlapping with the gate electrode layer 160 and the sidewall insulating layer 176 is almost occupied by the low-resistance region 127. Therefore, since the proportion of the low-resistance region 127 in the second region is lower than that in the third region, the second region has higher resistance than the third region and thus can substantially serve as an LDD region.

The above-described method for forming the low-resistance region 127 is extremely inexpensive and easy because an ion addition treatment or the like is not performed. Furthermore, the resistance can be reduced as compared with the case where the low-resistance region 127 is formed by an ion addition treatment. Therefore, the use of one embodiment of the present invention can improve the productivity of the semiconductor device including a transistor.

Then, a third insulating film to be the insulating layer 180 is formed. The third insulating film can be formed in a manner similar to that of the insulating layer 110. It is desirable to perform planarization after the deposition of the third insulating film.

After that, the third insulating film is etched by a dry etching method to form an opening.

Subsequently, a third conductive film to be the conductive layer 190 is formed in the opening, and then planarization treatment is performed, whereby the conductive layer 190 is formed.

Next, a fourth conductive film to be the conductive layer 195 is formed over the conductive layer 190. To form the conductive layer 195, the fourth conductive film is processed by a photolithography method, a nanoimprinting method, or the like (see FIGS. 18A to 18C).

Accordingly, with one embodiment of the present invention, an extremely miniaturized transistor whose channel length is less than or equal to 100 inn, less than or equal to 30 inn, or less than or equal to 20 nm can be manufactured stably.

Note that heat treatment may be performed at any time as required in each step.

Although not mentioned in the above manufacturing method of the transistor, a region with even lower resistance may be provided by ion addition. For example, ions are added to the second region and the third region of the oxide semiconductor layer 122 using the gate electrode layer 160 as a mask. As a material to be added, hydrogen (H), fluorine (F), boron (B), phosphorus (P), helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), tungsten (W), aluminum (Al), molybdenum (Mo), indium (In), or the like can be used. The addition can be performed by an ion doping method, an ion implantation method, a plasma immersion ion implantation method, high-density plasma treatment, or the like. Note that an ion implantation method is preferable for miniaturization because addition of impurities other than the predetermined ion can be inhibited. A large area can be effectively treated by an ion doping method or a plasma immersion ion implantation method.

The ion accelerating voltage for the ion addition treatment is preferably controlled in accordance with the ion species and implantation depth. For example, the ion accelerating voltage can be higher than or equal to 0.5 kV and lower than or equal to 100 kV, or higher than or equal to 3 kV and lower than or equal to 60 kV. The dose of the ions is preferably greater than or equal to 1×10¹² ions/cm² and less than or equal to 1×10¹⁷ ions/cm², further preferably greater than or equal to 1×10¹³ ions/cm² and less than or equal to 5×10¹⁶ ions/cm².

By the ion addition treatment, oxygen vacancies are formed in the oxide semiconductor layer 122 and the oxide insulating layer 123, so that the low-resistance region 125 is formed in the second and third regions. Note that the low-resistance region 125 may be formed in the entire second region or in part of the second region.

By performing third heat treatment after the ion addition treatment, damage caused to a film during the ion addition treatment can be repaired. By the heat treatment, the material to which the ion is added can be diffused into the oxide insulating layer 121.

<Modification Example 1 of Transistor 10: Transistor 11>

A transistor 11 with a shape different from that of the transistor 10 illustrated in FIGS. 1A to 1C will be described with reference to FIGS. 19A to 19C.

FIGS. 19A to 19C are a top view and cross-sectional views of the transistor 11. FIG. 19A is a top view of the transistor 11 and FIGS. 19B and 19C are cross-sectional views taken along dashed-dotted line B1-B2 and dashed-dotted line B3-B4 in FIG. 19A, respectively.

The end portion of the low-resistance region 127 in the transistor 11 is wider than that in the transistor 10. As illustrated in FIGS. 19A to 19C, the low-resistance region 127 may be included only in the oxide semiconductor layer 122 or may be included also in the oxide insulating layer 123.

Like the transistor 10, the distance between a top surface of the low-resistance region 127 and a top surface of the substrate 100 may be the same as the distance between an upper end of the oxide semiconductor layer 122 and the top surface of the substrate 100 (see FIG. 20B), or may be larger or smaller than the distance between the upper end of the oxide semiconductor layer 122 and the top surface of the substrate 100.

With the structure of the transistor 11, a side wall portion in the channel length direction can also be protected. Accordingly, the reliability of the transistor can be improved.

<Modification Example 2 of Transistor 10: Transistor 12>

A transistor 12 with a shape different from that of the transistor 10 illustrated in FIGS. 1A to 1C will be described with reference to FIGS. 21A to 21C.

FIGS. 21A to 21C are a top view and cross-sectional views of the transistor 12. FIG. 21A is a top view of the transistor 12 and FIGS. 21B and 21C are cross-sectional views taken along dashed-dotted line C1-C2 and dashed-dotted line C3-C4 in FIG. 21A, respectively.

The transistor 12 is different from the transistor 10 in that a conductive layer 165 is provided.

<Conductive Layer 165>

The conductive layer 165 can be formed using a material similar to that of the gate electrode layer 160. Note that the conductive layer 165 may be a single layer or a stacked layer.

The conductive layer 165 can have a function similar to that of the gate electrode layer 160. The conductive layer 165 and the gate electrode layer 160 may be configured to be applied with the same potential or different potentials.

In the transistor 12 including the conductive layer 165, the insulating layer 110 can have a structure and a function similar to those of the gate insulating layer 150.

With the structure of the transistor 12, electrical characteristics (e.g., threshold voltage) of a transistor can be controlled.

<Modification Example 3 of Transistor 10: Transistor 13>

A transistor 13 with a shape different from that of the transistor 10 illustrated in FIGS. 22A to 22C will be described with reference to FIGS. 22A to 22C.

FIGS. 22A to 22C are a top view and cross-sectional views of the transistor 13. FIG. 22A is a top view of the transistor 13 and FIGS. 22B and 22C are cross-sectional views taken along dashed-dotted line D1-D2 and dashed-dotted line D3-D4 in FIG. 22A, respectively.

The transistor 13 is different from the transistor 10 in that insulating layers 170 and 172 are provided.

<Insulating Layer 170>

The insulating layer 170 can contain oxygen (O), nitrogen (N), fluorine (F), aluminum (Al), magnesium (Mg), silicon (Si), gallium (Ga), germanium (Ge), yttrium (Y), zirconium (Zr), lanthanum (La), neodymium (Nd), hafnium (Hf), tantalum (Ta), titanium (Ti), or the like. For example, the insulating layer 170 can contain one or more of aluminum oxide (AlO_(x)), magnesium oxide (MgO_(x)), silicon oxide (SiO_(x)), silicon oxynitride (SiO_(x)N_(y)), silicon nitride oxide (SiN_(x)O_(y)), silicon nitride (SiN_(x)), gallium oxide (GaO_(x)), germanium oxide (GeO_(x)), yttrium oxide (YO_(x)), zirconium oxide (ZrO_(x)), lanthanum oxide (LaO_(x)), neodymium oxide (NdO_(x)), hafnium oxide (HfO_(x)), and tantalum oxide (TaO_(x)).

An aluminum oxide (AlO_(x)) film is preferably included in the insulating layer 170. The aluminum oxide film can prevent the passage of both oxygen and impurities such as hydrogen and moisture. Thus, during and after the manufacturing process of the transistor, the aluminum oxide film can suitably function as a protective film that has effects of preventing entry of impurities such as hydrogen and moisture, which cause variations in the electrical characteristics of the transistor, into the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123, preventing release of oxygen, which is a main component, from the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123, and preventing unnecessary release of oxygen from the insulating layer 110.

The insulating layer 170 is preferably a film having oxygen supply capability. In the deposition of the insulating layer 170, a mixed layer is formed at an interface with a different oxide layer and oxygen is supplied to the mixed layer or the different oxide layer. The oxygen is diffused into the oxide semiconductor layer by heat treatment performed after that, and the oxygen can fill oxygen vacancies in the oxide semiconductor layer; therefore, the transistor characteristics (e.g., threshold voltage and reliability) can be improved.

Further, the insulating layer 170 may be a single layer or a stacked layer. Alternatively, another insulating layer may be provided over or under the insulating layer. For example, an insulating film containing one or more of magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide can be used. The insulating layer preferably contains oxygen more than that in the stoichiometric composition. Oxygen released from the insulating layer can be diffused into the channel formation region in the oxide semiconductor layer 122 through the gate insulating layer 150 or the insulating layer 110, so that oxygen vacancies formed in the channel formation region can be filled with the oxygen. In this manner, stable electrical characteristics of the transistor can be achieved.

<Insulating Layer 172>

The insulating layer 172 can contain oxygen (O), nitrogen (N), fluorine (F), aluminum (Al), magnesium (Mg), silicon (Si), gallium (Ga), germanium (Ge), yttrium (Y), zirconium (Zr), lanthanum (La), neodymium (Nd), hafnium (Hf), tantalum (Ta), titanium (Ti), or the like. For example, the insulating layer 172 can contain one or more of aluminum oxide (AlO_(x)), magnesium oxide (MgO_(x)), silicon oxide (SiO_(x)), silicon oxynitride (SiO_(x)N_(y)), silicon nitride oxide (SiN_(x)O_(y)), silicon nitride (SiN_(x)), gallium oxide (GaO_(x)), germanium oxide (GeO_(x)), yttrium oxide (YO_(x)), zirconium oxide (ZrO_(x)), lanthanum oxide (LaO_(x)), neodymium oxide (NdO_(x)), hafnium oxide (HfO_(x)), and tantalum oxide (TaO_(x)). The insulating layer 172 may be a stack of any of the above materials.

An aluminum oxide film is preferably included in the insulating layer 172. The aluminum oxide film can prevent the passage of both oxygen and impurities such as hydrogen and moisture. Thus, during and after the manufacturing process of the transistor, the aluminum oxide film can suitably function as a protective film that has effects of preventing entry of impurities such as hydrogen and moisture, which cause variations in the electrical characteristics of the transistor, into the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123, preventing release of oxygen, which is a main component, from the oxide insulating layer 121, the oxide semiconductor layer 122, and the oxide insulating layer 123, and preventing release of oxygen from the insulating layer 110.

The insulating layer 172 can function as a protective film. The insulating layer 172 can protect the gate insulating layer 150 against plasma damage. This can prevent an electron trap from being formed in the vicinity of a channel.

<Manufacturing Method of Transistor 13>

Part of a manufacturing method of the transistor 13 will be described with reference to FIGS. 23A to 23C. For steps similar to those of the manufacturing method of the transistor 10, the description of the manufacturing method is referred to.

<Formation of Insulating Layer 172>

The insulating layer 172 is deposited over the oxide semiconductor layer 122, the sidewall insulating layer 176, and the gate electrode layer 160. The insulating layer 172 is preferably deposited by a metal organic chemical vapor deposition (MOCVD) method and an atomic layer deposition (ALD) method. Accordingly, damage to the oxide semiconductor layer 122 and the gate insulating layer 150 can be inhibited and oxidation of the gate electrode layer can be inhibited.

The thickness of the insulating layer 172 is preferably greater than or equal to 1 nm and less than or equal to 30 nm, and further preferably greater than or equal to 3 nm and less than or equal to 10 nm.

An ion may be added to the oxide semiconductor layer 122 after the insulating layer 172 is deposited. Accordingly, the low-resistance region 127 can be formed while damage to the oxide semiconductor layer 122 in the ion addition treatment can be reduced.

The insulating layer 172 may be used as deposited or may be processed by a lithography method, a nanoimprinting method, a dry etching method, or the like.

<Formation of Insulating Layer 170>

Next, the insulating layer 170 is formed over the insulating layer 172. The insulating layer 170 may be a single layer or a stacked layer. The insulating layer 170 can be formed using a material, a method, and the like similar to those of the insulating layer 110.

The insulating layer 170 is preferably an aluminum oxide film deposited by a sputtering method. A sputtering gas used for depositing the aluminum oxide film preferably contains an oxygen gas. The oxygen gas is contained at 1 vol % or more and 100 vol % or less, preferably 4 vol % or more and 100 vol % or less and further preferably 10 vol % or more and 100 vol % or less. When oxygen is contained at 1 vol % or more, excess oxygen can be supplied to the insulating layer or insulating layers in contact with the insulating layer. Furthermore, oxygen can be added to the layers in contact with the layer.

For example, the insulating layer 170 having a thickness from 20 nm to 40 nm can be deposited using aluminum oxide as a sputtering target and a sputtering gas that contains an oxygen gas at 50 vol %.

Next, heat treatment is preferably performed. The temperature of the heat treatment is typically higher than or equal to 150° C. and lower than the strain point of the substrate, preferably higher than or equal to 250° C. and lower than or equal to 500° C. and further preferably higher than or equal to 300° C. and lower than or equal to 450° C. By the heat treatment, oxygen 173 added to an insulating layer (e.g., the insulating layer 110) is diffused and moved to the oxide semiconductor layer 122, and oxygen vacancies in the oxide semiconductor layer 122 can be filled with the oxygen (see FIGS. 23A to 23C).

For example, the heat treatment can be performed at 400° C. in an oxygen atmosphere for one hour.

Note that heat treatment may be performed in other processes. Through heat treatment, defects in a film can be repaired, and the density of interface states can be reduced.

<Oxygen Addition>

Oxygen is not necessarily added through the insulating layer 170. Oxygen may be added to the insulating layer 110, the first oxide insulating film, and the oxide insulating layer 123, or another insulating layer. As the oxygen that is added, at least one selected from oxygen radicals, oxygen atoms, oxygen atomic ions, oxygen molecular ions, and the like is used. As a method for adding the oxygen, an ion doping method, an ion implantation method, a plasma immersion ion implantation method, or the like can be used.

In the case of using an ion implantation method as the method for adding oxygen, oxygen atomic ions or oxygen molecular ions can be used. The use of oxygen molecular ions can reduce damage to a film to which oxygen is added. Oxygen molecular ions are broken down into oxygen atomic ions at the surface of the film to which oxygen is added, and the oxygen atomic ions are added. Since energy for breaking oxygen molecules down into oxygen atoms is used, the energy per oxygen atomic ion in the case of adding oxygen molecular ions to the film to which oxygen is added is lower than that in the case of adding oxygen atomic ions to the film to which oxygen is added. Therefore, damage to the film to which oxygen is added can be reduced.

By using oxygen molecular ions, the energy of each oxygen atomic ion injected to the film to which oxygen is added is lowered, which makes the injected oxygen atomic ion be positioned in a shallow region. Accordingly, oxygen atoms easily move by later heat treatment, so that more oxygen can be supplied to the oxide semiconductor film.

In the case of injecting oxygen molecular ions, the energy per oxygen atomic ion is low as compared with the case of injecting oxygen atomic ions. Thus, by using oxygen molecular ions for injection, the acceleration voltage can be increased and throughput can be increased. Moreover, by using oxygen molecular ions for injection, the dose can be half of the amount that is necessary in the case of using oxygen atomic ions. As a result, throughput can be increased.

In the case of adding oxygen to the film to which oxygen is added, it is preferable that oxygen be added to the film to which oxygen is added so that a peak of the concentration profile of oxygen atomic ions is located in the film to which oxygen is added. In that case, the acceleration voltage for injection can be lowered as compared with the case where oxygen atomic ions are injected, and damage to the film to which oxygen is added can be reduced. In other words, defects in the film to which oxygen is added can be reduced, suppressing variations in electrical characteristics of the transistor. Furthermore, in the case where oxygen is added to the film to which oxygen is added so that the amount of added oxygen atoms at the interface between the insulating layer 110 and the oxide insulating layer 121 is less than 1×10²¹ atoms/cm³, less than 1×10²⁰ atoms/cm³, or less than 1×10¹⁹ atoms/cm³, the amount of oxygen added to the insulating layer 110 can be reduced. As a result, damage to the film to which oxygen is added can be reduced, suppressing variations in electrical characteristics of the transistor.

Plasma treatment (plasma immersion ion implantation method) in which the film to which oxygen is added is exposed to plasma generated in an atmosphere containing oxygen may be performed to add oxygen to the film to which oxygen is added. As an example of the atmosphere containing oxygen, an atmosphere containing an oxidation gas such as oxygen, ozone, dinitrogen monoxide, or nitrogen dioxide can be given. Note that it is preferable that the film to which oxygen is added be exposed to plasma generated in a state where bias is applied to the substrate 100 side because the amount of oxygen added to the film to which oxygen is added can be increased. As an example of an apparatus with which such plasma treatment is performed, an ashing apparatus or the like is given.

For example, oxygen molecular ions can be added to the insulating layer 110 by an ion implantation method with a dose of 2×10¹⁶/cm² at an acceleration voltage of 60 kV.

The above process is applicable to the transistor 10 and other transistors.

In this manner, the density of localized states of the oxide semiconductor film is lowered, and thus a transistor with excellent electrical characteristics can be manufactured. In addition, a highly reliable transistor in which variations in electrical characteristics with time or variations in electrical characteristics due to a stress test are reduced can be manufactured.

Note that this embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

Embodiment 2

<Structure of Oxide Semiconductor>

The structure of an oxide semiconductor is described below.

An oxide semiconductor is classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of a non-single-crystal oxide semiconductor include a c-axis aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor.

From another perspective, an oxide semiconductor is classified into an amorphous oxide semiconductor and a crystalline oxide semiconductor. Examples of a crystalline oxide semiconductor include a single crystal oxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor, and an nc-OS.

An amorphous structure is generally thought to be isotropic and have no non-uniform structure, to be metastable and not have fixed positions of atoms, to have a flexible bond angle, and to have a short-range order but have no long-range order, for example.

In other words, a stable oxide semiconductor cannot be regarded as a completely amorphous oxide semiconductor. Moreover, an oxide semiconductor that is not isotropic (e.g., an oxide semiconductor that has a periodic structure in a microscopic region) cannot be regarded as a completely amorphous oxide semiconductor. In contrast, an a-like OS, which is not isotropic, has an unstable structure that contains a void. Because of its instability, an a-like OS has physical properties similar to those of an amorphous oxide semiconductor.

<CAAC-OS>

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axis aligned crystal parts (also referred to as pellets).

Analysis of a CAAC-OS by X-ray diffraction (XRD) is described. For example, when the structure of a CAAC-OS including an InGaZnO₄ crystal that is classified into the space group R-3m is analyzed by an out-of-plane method, a peak appears at a diffraction angle (2θ) of around 31° as shown in FIG. 24A. This peak is derived from the (009) plane of the InGaZnO₄ crystal, which indicates that crystals in the CAAC-OS have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to a surface over which the CAAC-OS film is formed (also referred to as a formation surface) or the top surface of the CAAC-OS film. Note that a peak sometimes appears at a 2θ of around 36° in addition to the peak at a 2θ of around 31°. The peak at a 2θ of around 36° is derived from a crystal structure that is classified into the space group Fd-3m; thus, this peak is preferably not exhibited in a CAAC-OS.

On the other hand, in structural analysis of the CAAC-OS by an in-plane method in which an X-ray is incident on the CAAC-OS in a direction parallel to the formation surface, a peak appears at a 2θ of around 56°. This peak is attributed to the (110) plane of the InGaZnO₄ crystal. When analysis (φ scan) is performed with 2θ fixed at around 56° and with the sample rotated using a normal vector to the sample surface as an axis (φ axis), as shown in FIG. 24B, a peak is not clearly observed. In contrast, in the case where single crystal InGaZnO₄ is subjected to φ scan with 2θ fixed at around 56°, as shown in FIG. 24C, six peaks which are derived from crystal planes equivalent to the (110) plane are observed. Accordingly, the structural analysis using XRD shows that the directions of a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. For example, when an electron beam with a probe diameter of 300 nm is incident on a CAAC-OS including an InGaZnO₄ crystal in a direction parallel to the formation surface of the CAAC-OS, a diffraction pattern (also referred to as a selected-area electron diffraction pattern) shown in FIG. 24D can be obtained. In this diffraction pattern, spots derived from the (009) plane of an InGaZnO₄ crystal are included. Thus, the electron diffraction also indicates that pellets included in the CAAC-OS have c-axis alignment and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS. Meanwhile, FIG. 24E shows a diffraction pattern obtained in such a manner that an electron beam with a probe diameter of 300 nm is incident on the same sample in a direction perpendicular to the sample surface. As shown in FIG. 24E, a ring-like diffraction pattern is observed. Thus, the electron diffraction using an electron beam with a probe diameter of 300 nm also indicates that the a-axes and b-axes of the pellets included in the CAAC-OS do not have regular orientation. The first ring in FIG. 24E is considered to be derived from the (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal. The second ring in FIG. 24E is considered to be derived from the (110) plane and the like.

In a combined analysis image (also referred to as a high-resolution TEM image) of a bright-field image and a diffraction pattern of a CAAC-OS, which is obtained using a transmission electron microscope (TEM), a plurality of pellets can be observed. However, even in the high-resolution TEM image, a boundary between pellets, that is, a grain boundary is not clearly observed in some cases. Thus, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur.

FIG. 25A shows a high-resolution TEM image of a cross section of the CAAC-OS which is observed from a direction substantially parallel to the sample surface. The high-resolution TEM image is obtained with a spherical aberration corrector function. The high-resolution TEM image obtained with a spherical aberration corrector function is particularly referred to as a Cs-corrected high-resolution TEM image. The Cs-corrected high-resolution TEM image can be observed with, for example, an atomic resolution analytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 25A shows pellets in which metal atoms are arranged in a layered manner. FIG. 25A proves that the size of a pellet is greater than or equal to 1 nm or greater than or equal to 3 nm. Therefore, the pellet can also be referred to as a nanocrystal (nc). Furthermore, the CAAC-OS can also be referred to as an oxide semiconductor including c-axis aligned nanocrystals (CANC). A pellet reflects unevenness of a formation surface or a top surface of the CAAC-OS, and is parallel to the formation surface or the top surface of the CAAC-OS.

FIGS. 25B and 25C show Cs-corrected high-resolution TEM images of a plane of the CAAC-OS observed from a direction substantially perpendicular to the sample surface. FIGS. 25D and 25E are images obtained through image processing of FIGS. 25B and 25C. The method of image processing is as follows. The image in FIG. 25B is subjected to fast Fourier transform (FFT), so that an FFT image is obtained. Then, mask processing is performed such that a range of from 2.8 nm⁻¹ to 5.0 nm⁻¹ from the origin in the obtained FFT image remains. After the mask processing, the FFT image is processed by inverse fast Fourier transform (IFFT) to obtain a processed image. The image obtained in this manner is called an FFT filtering image. The FFT filtering image is a Cs-corrected high-resolution TEM image from which a periodic component is extracted, and shows a lattice arrangement.

In FIG. 25D, a portion where a lattice arrangement is broken is denoted with a dashed line. A region surrounded by a dashed line is one pellet. The portion denoted with the dashed line is a junction of pellets. The dashed line draws a hexagon, which means that the pellet has a hexagonal shape. Note that the shape of the pellet is not always a regular hexagon but is a non-regular hexagon in many cases.

In FIG. 25E, a dotted line denotes a portion where the direction of a lattice arrangement changes between a region with a regular lattice arrangement and another region with a regular lattice arrangement, and a dashed line denotes the change in the direction of the lattice arrangement. A clear crystal grain boundary cannot be observed even in the vicinity of the dotted line. When a lattice point in the vicinity of the dotted line is regarded as a center and surrounding lattice points are joined, a distorted hexagon, pentagon, and/or heptagon can be formed, for example. That is, a lattice arrangement is distorted so that formation of a crystal grain boundary is inhibited. This is probably because the CAAC-OS can tolerate distortion owing to a low density of the atomic arrangement in an a-b plane direction, an interatomic bond distance changed by substitution of a metal element, and the like.

As described above, the CAAC-OS has c-axis alignment, its pellets (nanocrystals) are connected in an a-b plane direction, and the crystal structure has distortion. For this reason, the CAAC-OS can also be referred to as an oxide semiconductor including a c-axis-aligned a-b-plane-anchored (CAA) crystal.

The CAAC-OS is an oxide semiconductor with high crystallinity. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has small amounts of impurities and defects (e.g., oxygen vacancies).

Note that the impurity means an element other than the main components of the oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. For example, an element (specifically, silicon or the like) having higher strength of bonding to oxygen than a metal element included in an oxide semiconductor extracts oxygen from the oxide semiconductor, which results in disorder of the atomic arrangement and reduced crystallinity of the oxide semiconductor. A heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities or defects might be changed by light, heat, or the like. Impurities contained in the oxide semiconductor might serve as carrier traps or carrier generation sources, for example. For example, an oxygen vacancy in the oxide semiconductor might serve as a carrier trap or serve as a carrier generation source when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancies is an oxide semiconductor with low carrier density (specifically, lower than 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, and further preferably lower than 1×10¹⁰/cm³, and is higher than or equal to 1×10⁻⁹/cm³). Such an oxide semiconductor is referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. A CAAC-OS has a low impurity concentration and a low density of defect states. Thus, the CAAC-OS can be referred to as an oxide semiconductor having stable characteristics.

<nc-OS>

Next, an nc-OS is described.

Analysis of an nc-OS by XRD is described. When the structure of an nc-OS is analyzed by an out-of-plane method, a peak indicating orientation does not appear. That is, a crystal of an nc-OS does not have orientation.

For example, when an electron beam with a probe diameter of 50 nm is incident on a 34-nm-thick region of thinned nc-OS including an InGaZnO₄ crystal in a direction parallel to the formation surface, a ring-shaped diffraction pattern (a nanobeam electron diffraction pattern) shown in FIG. 26A is observed. FIG. 26B shows a diffraction pattern obtained when an electron beam with a probe diameter of 1 nm is incident on the same sample. As shown in FIG. 26B, a plurality of spots are observed in a ring-like region. In other words, ordering in an nc-OS is not observed with an electron beam with a probe diameter of 50 nm but is observed with an electron beam with a probe diameter of 1 nm.

Furthermore, an electron diffraction pattern in which spots are arranged in an approximately hexagonal shape is observed in some cases as shown in FIG. 26C when an electron beam having a probe diameter of 1 nm is incident on a region with a thickness of less than 10 nm. This means that an nc-OS has a well-ordered region, i.e., a crystal, in the range of less than 10 nm in thickness. Note that an electron diffraction pattern having regularity is not observed in some regions because crystals are aligned in various directions.

FIG. 26D shows a Cs-corrected high-resolution TEM image of a cross section of an nc-OS observed from the direction substantially parallel to the formation surface. In a high-resolution TEM image, an nc-OS has a region in which a crystal part is observed, such as the part indicated by additional lines in FIG. 26D, and a region in which a crystal part is not clearly observed. In most cases, the size of a crystal part included in the nc-OS is greater than or equal to 1 nm and less than or equal to 10 nm, in particular, greater than or equal to 1 nm and less than or equal to 3 nm. Note that an oxide semiconductor including a crystal part whose size is greater than 10 nm and less than or equal to 100 nm is sometimes referred to as a microcrystalline oxide semiconductor. In a high-resolution TEM image of the nc-OS, for example, a grain boundary is not clearly observed in some cases. Note that there is a possibility that the origin of the nanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may be referred to as a pellet in the following description.

As described above, in the nc-OS, a microscopic region (e.g., a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. There is no regularity of crystal orientation between different pellets in the nc-OS. Thus, the orientation of the whole film is not ordered. Accordingly, the nc-OS cannot be distinguished from an a-like OS or an amorphous oxide semiconductor, depending on an analysis method.

Since there is no regularity of crystal orientation between the pellets (nanocrystals) as mentioned above, the nc-OS can also be referred to as an oxide semiconductor including random aligned nanocrystals (RANC) or an oxide semiconductor including non-aligned nanocrystals (NAN C).

The nc-OS is an oxide semiconductor that has high regularity as compared to an amorphous oxide semiconductor. Therefore, the nc-OS is likely to have a lower density of defect states than an a-like OS and an amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different pellets in the nc-OS. Therefore, the nc-OS has a higher density of defect states than the CAAC-OS.

<a-Like OS>

An a-like OS has a structure intermediate between those of the nc-OS and the amorphous oxide semiconductor.

FIGS. 27A and 27B are high-resolution cross-sectional TEM images of an a-like OS. FIG. 27A is the high-resolution cross-sectional TEM image of the a-like OS at the start of the electron irradiation. FIG. 27B is the high-resolution cross-sectional TEM image of a-like OS after the electron (e⁻) irradiation at 4.3×10⁸ e⁻/nm². FIGS. 27A and 27B show that stripe-like bright regions extending vertically are observed in the a-like OS from the start of the electron irradiation. It can be also found that the shape of the bright region changes after the electron irradiation. Note that the bright region is presumably a void or a low-density region.

The a-like OS has an unstable structure because it contains a void. To verify that an a-like OS has an unstable structure as compared to a CAAC-OS and an nc-OS, a change in structure caused by electron irradiation is described below.

An a-like OS, an nc-OS, and a CAAC-OS are prepared as samples. Each of the samples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample is obtained. The high-resolution cross-sectional TEM images show that all the samples have crystal parts.

It is known that a unit cell of an InGaZnO₄ crystal has a structure in which nine layers including three In—O layers and six Ga—Zn—O layers are stacked in the c-axis direction. The distance between the adjacent layers is equivalent to the lattice spacing on the (009) plane (also referred to as d value). The value is calculated to be 0.29 nm from crystal structural analysis. Accordingly, a portion where the spacing between lattice fringes is greater than or equal to 0.28 nm and less than or equal to 0.30 nm is regarded as a crystal part of InGaZnO₄ in the following description. Each of lattice fringes corresponds to the a-b plane of the InGaZnO₄ crystal.

FIG. 28 shows change in the average size of crystal parts (at 22 points to 30 points) in each sample. Note that the crystal part size corresponds to the length of a lattice fringe. FIG. 28 indicates that the crystal part size in the a-like OS increases with an increase in the cumulative electron dose in obtaining TEM images, for example. As shown in FIG. 28, a crystal part of approximately 1.2 nm (also referred to as an initial nucleus) at the start of TEM observation grows to a size of approximately 1.9 nm at a cumulative electron (e⁻) dose of 4.2×10⁸ e⁻/nm². In contrast, the crystal part size in the nc-OS and the CAAC-OS shows little change from the start of electron irradiation to a cumulative electron dose of 4.2×10⁸ e⁻/nm². As shown in FIG. 28, the crystal part sizes in an nc-OS and a CAAC-OS are approximately 1.3 nm and approximately 1.8 nm, respectively, regardless of the cumulative electron dose. For the electron beam irradiation and TEM observation, a Hitachi H-9000NAR transmission electron microscope was used. The conditions of electron beam irradiation were as follows: the accelerating voltage was 300 kV; the current density was 6.7×10⁵ e⁻/(nm²·s); and the diameter of irradiation region was 230 nm.

In this manner, growth of the crystal part in the a-like OS is sometimes induced by electron irradiation. In contrast, in the nc-OS and the CAAC-OS, growth of the crystal part is hardly induced by electron irradiation. Therefore, the a-like OS has an unstable structure as compared to the nc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS because it contains a void. Specifically, the density of the a-like OS is higher than or equal to 78.6% and lower than 92.3% of the density of the single crystal oxide semiconductor having the same composition. The density of each of the nc-OS and the CAAC-OS is higher than or equal to 92.3% and lower than 100% of the density of the single crystal oxide semiconductor having the same composition. Note that it is difficult to deposit an oxide semiconductor having a density of lower than 78% of the density of the single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with a rhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of the a-like OS is higher than or equal to 5.0 g/cm³ and lower than 5.9 g/cm³. For example, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of each of the nc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lower than 6.3 g/cm³.

Note that in the case where an oxide semiconductor having a certain composition does not exist in a single crystal structure, single crystal oxide semiconductors with different compositions are combined at an adequate ratio, which makes it possible to calculate density equivalent to that of a single crystal oxide semiconductor with the desired composition. The density of a single crystal oxide semiconductor having the desired composition can be calculated using a weighted average according to the combination ratio of the single crystal oxide semiconductors with different compositions. Note that it is preferable to use as few kinds of single crystal oxide semiconductors as possible to calculate the density.

As described above, oxide semiconductors have various structures and various properties. Note that an oxide semiconductor may be a stacked layer film including two or more films of an amorphous oxide semiconductor, an a-like OS, an nc-OS, and a CAAC-OS, for example.

Embodiment 3

In this embodiment, an example of a circuit including the transistor of one embodiment of the present invention will be described with reference to drawings.

<Cross-Sectional Structure>

FIG. 29A is a cross-sectional view of a semiconductor device of one embodiment of the present invention. In FIG. 29A, X1-X2 direction and Y1-Y2 direction represent a channel length direction and a channel width direction, respectively. The semiconductor device illustrated in FIG. 29A includes a transistor 2200 using a first semiconductor material in a lower portion and a transistor 2100 using a second semiconductor material in an upper portion. In FIG. 29A, an example is described in which the transistor described in the above embodiment as an example is used as the transistor 2100 using the second semiconductor material. A cross-sectional view of the transistors in a channel length direction is on the left side of a dashed-dotted line, and a cross-sectional view of the transistors in a channel width direction is on the right side of the dashed-dotted line.

Here, the first semiconductor material and the second semiconductor material are preferably materials having different band gaps. For example, the first semiconductor material can be a semiconductor material other than an oxide semiconductor (examples of such a semiconductor material include silicon (including strained silicon), germanium, silicon germanium, silicon carbide, gallium arsenide, aluminum gallium arsenide, indium phosphide, gallium nitride, and an organic semiconductor), and the second semiconductor material can be an oxide semiconductor. A transistor using a material other than an oxide semiconductor, such as single crystal silicon, can operate at high speed easily. In contrast, a transistor using an oxide semiconductor and described in the above embodiment as an example can have a small subthreshold value (S value) and a minute structure. Furthermore, the transistor can operate at a high speed because of its high switching speed and has low leakage current because of its low off-state current.

The transistor 2200 may be either an n-channel transistor or a p-channel transistor, and an appropriate transistor may be used in accordance with a circuit. Furthermore, the specific structure of the semiconductor device, such as the material or the structure used for the semiconductor device, is not necessarily limited to those described here except for the use of the transistor of one embodiment of the present invention which uses an oxide semiconductor.

FIG. 29A illustrates a structure in which the transistor 2100 is provided over the transistor 2200 with insulators 2201 and 2207 provided therebetween. A plurality of wirings 2202 are provided between the transistor 2200 and the transistor 2100. Furthermore, wirings and electrodes provided over and under the insulators are electrically connected to each other through a plurality of plugs 2203 embedded in the insulators. An insulator 2204 covering the transistor 2100 and a wiring 2205 over the insulator 2204 are provided.

The stack of the two kinds of transistors reduces the area occupied by the circuit, allowing a plurality of circuits to be highly integrated.

Here, in the case where a silicon-based semiconductor material is used for the transistor 2200 provided in a lower portion, hydrogen in an insulator provided in the vicinity of the semiconductor film of the transistor 2200 terminates dangling bonds of silicon; accordingly, the reliability of the transistor 2200 can be improved. Meanwhile, in the case where an oxide semiconductor is used for the transistor 2100 provided in an upper portion, hydrogen in an insulator provided in the vicinity of the semiconductor film of the transistor 2100 becomes a factor of generating carriers in the oxide semiconductor; thus, the reliability of the transistor 2100 might be decreased. Therefore, in the case where the transistor 2100 using an oxide semiconductor is provided over the transistor 2200 using a silicon-based semiconductor material, providing the insulator 2207 having a function of preventing diffusion of hydrogen between the transistors 2100 and 2200 is particularly effective. The insulator 2207 makes hydrogen remain in the lower portion, thereby improving the reliability of the transistor 2200. In addition, since the insulator 2207 suppresses diffusion of hydrogen from the lower portion to the upper portion, the reliability of the transistor 2100 can also be improved.

The insulator 2207 can be, for example, formed using aluminum oxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttrium oxynitride, hafnium oxide, hafnium oxynitride, or yttria-stabilized zirconia (YSZ).

Furthermore, a blocking film having a function of preventing diffusion of hydrogen is preferably formed over the transistor 2100 to cover the transistor 2100 including an oxide semiconductor film. For the blocking film, a material that is similar to that of the insulator 2207 can be used, and in particular, an aluminum oxide film is preferably used. The aluminum oxide film has a high shielding (blocking) effect of preventing penetration of both oxygen and impurities such as hydrogen and moisture. Thus, by using the aluminum oxide film as the blocking film covering the transistor 2100, release of oxygen from the oxide semiconductor film included in the transistor 2100 can be prevented and entry of water and hydrogen into the oxide semiconductor film can be prevented. Note that as the block film, the insulator 2204 having a stacked-layer structure may be used, or the block film may be provided under the insulator 2204.

Note that the transistor 2200 can be a transistor of various types without being limited to a planar type transistor. For example, the transistor 2200 can be a fin-type transistor, a tri-gate transistor, or the like. An example of a cross-sectional view in this case is illustrated in FIG. 29D. An insulator 2212 is provided over a semiconductor substrate 2211. The semiconductor substrate 2211 includes a projecting portion with a thin tip (also referred to a fin). Note that an insulator may be provided over the projecting portion. The projecting portion does not necessarily have the thin tip; a projecting portion with a cuboid-like projecting portion and a projecting portion with a thick tip are permitted, for example. A gate insulator 2214 is provided over the projecting portion of the semiconductor substrate 2211, and a gate electrode 2213 is provided over the gate insulator 2214. Source and drain regions 2215 are formed in the semiconductor substrate 2211. Note that here is shown an example in which the semiconductor substrate 2211 includes the projecting portion; however, a semiconductor device of one embodiment of the present invention is not limited thereto. For example, a semiconductor region having a projecting portion may be formed by processing an SOI substrate.

<Circuit Configuration Example>

In the above structure, electrodes of the transistors 2100 and 2200 can be connected as appropriate; thus, a variety of circuits can be formed. Examples of circuit configurations which can be achieved by using a semiconductor device of one embodiment of the present invention are described below.

<CMOS Inverter Circuit>

A circuit diagram in FIG. 29B shows a configuration of a CMOS inverter in which the p-channel transistor 2200 and the n-channel transistor 2100 are connected to each other in series and in which gates of them are connected to each other.

<CMOS Analog Switch>

A circuit diagram in FIG. 29C shows a configuration in which sources of the transistors 2100 and 2200 are connected to each other and drains of the transistors 2100 and 2200 are connected to each other. In FIG. 29A, X1-X2 direction and Y1-Y2 direction represent a channel length direction and a channel width direction, respectively. With such a configuration, the transistors can function as a CMOS analog switch.

<Memory Device Example>

An example of a semiconductor device (memory device) which includes the transistor of one embodiment of the present invention, which can retain stored data even when not powered, and which has an unlimited number of write cycles is shown in FIGS. 30A to 30C.

The semiconductor device illustrated in FIG. 30A includes a transistor 3200 using a first semiconductor material, a transistor 3300 using a second semiconductor material, and a capacitor 3400. As the transistor 3300, the transistor described in Embodiment 1 can be used.

FIG. 30B is a cross-sectional view of the semiconductor device shown in FIG. 30A. The semiconductor device in the cross-sectional view has a structure in which the transistor 3300 is provided with a back gate; however, a structure without a back gate may be employed.

The transistor 3300 is a transistor in which a channel is formed in a semiconductor including an oxide semiconductor. Since the off-state current of the transistor 3300 is low, stored data can be retained for a long period. In other words, power consumption can be sufficiently reduced because a semiconductor memory device in which refresh operation is unnecessary or the frequency of refresh operation is extremely low can be provided.

In FIG. 30A, a first wiring 3001 is electrically connected to a source electrode of the transistor 3200. A second wiring 3002 is electrically connected to a drain electrode of the transistor 3200. A third wiring 3003 is electrically connected to one of a source electrode and a drain electrode of the transistor 3300. A fourth wiring 3004 is electrically connected to a gate electrode of the transistor 3300. A gate electrode of the transistor 3200 is electrically connected to the other of the source electrode and the drain electrode of the transistor 3300 and one electrode of the capacitor 3400. The fifth wiring 3005 is electrically connected to the other electrode of the capacitor 3400. Although the capacitor 3400 is provided over the transistor 3300 in FIG. 30B, the capacitor 3400 may be provided between the transistor 3200 and the transistor 3300.

The semiconductor device in FIG. 30A has a feature that the potential of the gate electrode of the transistor 3200 can be retained, and thus enables writing, retaining, and reading of data as follows.

Writing and retaining of data are described. First, the potential of the fourth wiring 3004 is set to a potential at which the transistor 3300 is turned on, so that the transistor 3300 is turned on. Accordingly, the potential of the third wiring 3003 is supplied to the gate of the transistor 3200 and the capacitor 3400. That is, a predetermined charge is supplied to the gate of the transistor 3200 (writing). Here, one of two kinds of charges providing different potential levels (hereinafter referred to as a low-level charge and a high-level charge) is supplied. After that, the potential of the fourth wiring 3004 is set to a potential at which the transistor 3300 is turned off, so that the transistor 3300 is turned off. Thus, the charge supplied to the gate of the transistor 3200 is held (retaining).

Since the off-state current of the transistor 3300 is extremely low, the charge of the gate of the transistor 3200 is retained for a long time.

Next, reading of data is described. An appropriate potential (a reading potential) is supplied to the fifth wiring 3005 while a predetermined potential (a constant potential) is supplied to the first wiring 3001, whereby the potential of the second wiring 3002 varies depending on the amount of charge retained in the gate electrode of the transistor 3200. This is because in the case of using an n-channel transistor as the transistor 3200, an apparent threshold voltage V_(th) _(_) _(H) at the time when the high-level charge is given to the gate electrode of the transistor 3200 is lower than an apparent threshold voltage V_(th) _(_) _(L) at the time when the low-level charge is given to the gate electrode of the transistor 3200. Here, an apparent threshold voltage refers to the potential of the fifth wiring 3005 which is needed to turn on the transistor 3200. Thus, the potential of the fifth wiring 3005 is set to a potential V₀ which is between V_(th) _(_) _(H) and V_(th) _(_) _(L), whereby charge supplied to the gate electrode of the transistor 3200 can be determined. For example, in the case where the high-level charge is supplied to the gate of the transistor 3200 in writing and the potential of the fifth wiring 3005 is V₀ (>V_(th) _(_) _(H)), the transistor 3200 is turned on. In the case where the low-level charge is supplied to the gate of the transistor 3200 in writing, even when the potential of the fifth wiring 3005 is V₀ (<V_(th) _(_) _(L)), the transistor 3200 remains off. Thus, the data retained in the gate of the transistor 3200 can be read by determining the potential of the second wiring 3002.

Note that in the case where memory cells are arrayed to be used, it is necessary that only data of a desired memory cell be able to be read. For example, the fifth wiring 3005 of memory cells from which data is not read may be supplied with a potential at which the transistor 3200 is turned off regardless of the potential of the gate, that is, a potential lower than V_(th) _(_) _(H), whereby only data of a desired memory cell can be read. Alternatively, the fifth wiring 3005 of the memory cells from which data is not read may be supplied with a potential at which the transistor 3200 is turned on regardless of the potential of the gate, that is, a potential higher than V_(th) _(_) _(L), whereby only data of a desired memory cell can be read.

The semiconductor device shown in FIG. 30C is different from the semiconductor device shown in FIG. 30A in that the transistor 3200 is not provided. Also in this case, writing and holding of data can be performed in a manner similar to the above.

Next, reading of data is described. When the transistor 3300 is turned on, the third wiring 3003 which is in a floating state and the capacitor 3400 are electrically connected to each other, and the charge is redistributed between the third wiring 3003 and the capacitor 3400. As a result, the potential of the third wiring 3003 is changed. The amount of change in the potential of the third wiring 3003 varies depending on the potential of the first terminal of the capacitor 3400 (or the charge accumulated in the capacitor 3400).

For example, the potential of the third wiring 3003 after the charge redistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potential of the first terminal of the capacitor 3400, C is the capacitance of the capacitor 3400, C_(B) is the capacitance component of the third wiring 3003, and V_(B0) is the potential of the third wiring 3003 before the charge redistribution. Thus, it can be found that, assuming that the memory cell is in either of two states in which the potential of the first terminal of the capacitor 3400 is V₁ and V₀ (V₁>V₀), the potential of the third wiring 3003 in the case of retaining the potential V₁ (=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of the third wiring 3003 in the case of retaining the potential V₀ (=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with a predetermined potential, data can be read.

In this case, a transistor including the first semiconductor material may be used for a driver circuit for driving a memory cell, and a transistor including the second semiconductor material may be stacked over the driver circuit as the transistor 3300.

When including a transistor in which a channel formation region is formed using an oxide semiconductor and which has an extremely low off-state current, the semiconductor device described in this embodiment can retain stored data for an extremely long time. In other words, refresh operation becomes unnecessary or the frequency of the refresh operation can be extremely low, which leads to a sufficient reduction in power consumption. Moreover, stored data can be retained for a long time even when power is not supplied (note that a potential is preferably fixed).

Further, in the semiconductor device described in this embodiment, high voltage is not needed for writing data and there is no problem of deterioration of elements. Unlike in a conventional nonvolatile memory, for example, it is not necessary to inject and extract electrons into and from a floating gate; thus, a problem such as deterioration of a gate insulating layer is not caused. That is, the semiconductor device of the disclosed invention does not have a limit on the number of times data can be rewritten, which is a problem of a conventional nonvolatile memory, and the reliability thereof is drastically improved. Furthermore, data is written depending on the state of the transistor (on or off), whereby high-speed operation can be easily achieved.

With the use of the semiconductor device described in this embodiment, a memory device with low power consumption and high capacity (e.g., 1 terabit or more) can be manufactured.

Note that in this specification, it might be possible for those skilled in the art to constitute one embodiment of the invention even when portions to which all the terminals of an active element (e.g., a transistor or a diode), a passive element (e.g., a capacitor or a resistor), or the like are connected are not specified. In other words, one embodiment of the invention can be clear even when connection portions are not specified. Further, in the case where a connection portion is disclosed in this specification and the like, it can be determined that one embodiment of the invention in which a connection portion is not specified is disclosed in this specification and the like, in some cases. In particular, in the case where the number of portions to which the terminal is connected might be plural, it is not necessary to specify the portions to which the terminal is connected. Therefore, it might be possible to constitute one embodiment of the invention by specifying only portions to which some of terminals of an active element (e.g., a transistor or a diode), a passive element (e.g., a capacitor or a resistor), or the like are connected.

Note that in this specification, it might be possible for those skilled in the art to specify the invention when at least the connection portion of a circuit is specified. Alternatively, it might be possible for those skilled in the art to specify the invention when at least a function of a circuit is specified. In other words, when a function of a circuit is specified, one embodiment of the invention can be clear. Furthermore, it can be determined that one embodiment of the invention whose function is specified is disclosed in this specification and the like. Therefore, when a connection portion of a circuit is specified, the circuit is disclosed as one embodiment of the invention even when a function is not specified, and one embodiment of the invention can be constituted. Alternatively, when a function of a circuit is specified, the circuit is disclosed as one embodiment of the invention even when a connection portion is not specified, and one embodiment of the invention can be constituted.

Note that in this specification, in a diagram or a text described in one embodiment, it is possible to take out part of the diagram or the text and constitute an embodiment of the invention. Thus, in the case where a diagram or a text related to a certain portion is described, the context taken out from part of the diagram or the text is also disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. Therefore, for example, in a diagram or text in which one or more active elements (e.g., transistors or diodes), wirings, passive elements (e.g., capacitors or resistors), conductive layers, insulating layers, semiconductors, organic materials, inorganic materials, components, devices, operating methods, manufacturing methods, or the like are described, part of the diagram or the text is taken out, and one embodiment of the invention can be constituted. For example, from a circuit diagram in which N circuit elements (e.g., transistors or capacitors; N is an integer) are provided, it is possible to constitute one embodiment of the invention by taking out M circuit elements (e.g., transistors or capacitors; M is an integer, where M<N). As another example, it is possible to constitute one embodiment of the invention by taking out M layers (M is an integer, where M<N) from a cross-sectional view in which N layers (N is an integer) are provided. As another example, it is possible to constitute one embodiment of the invention by taking out M elements (M is an integer, where M<N) from a flow chart in which N elements (N is an integer) are provided.

<Imaging Device>

An imaging device of one embodiment of the present invention is described below.

FIG. 31A is a plan view illustrating an example of an imaging device 200 of one embodiment of the present invention. The imaging device 200 includes a pixel portion 210 and peripheral circuits for driving the pixel portion 210 (peripheral circuits 260, 270, 280, and 290). The pixel portion 210 includes a plurality of pixels 211 arranged in a matrix with p rows and q columns (p and q are each an integer greater than or equal to 2). The peripheral circuits 260, 270, 280, and 290 are each connected to a plurality of pixels 211 and each have a function of supplying a signal for driving the plurality of pixels 211. In this specification, in some cases, a “peripheral circuit” or a “driver circuit” indicates all of the peripheral circuits 260, 270, 280, and 290. For example, the peripheral circuit 260 can be regarded as part of the peripheral circuit.

The peripheral circuit includes at least one of a logic circuit, a switch, a buffer, an amplifier circuit, and a converter circuit. The peripheral circuit may be provided over a substrate where the pixel portion 210 is formed. A semiconductor device such as an IC chip may be used as part or the whole of the peripheral circuit. Note that as the peripheral circuit, one or more of the peripheral circuits 260, 270, 280, and 290 may be omitted.

As illustrated in FIG. 31B, the pixels 211 may be provided to be inclined in the pixel portion 210 included in the imaging device 200. When the pixels 211 are obliquely arranged, the distance between pixels (pitch) can be shortened in the row direction and the column direction. Accordingly, the quality of an image taken with the imaging device 200 can be improved.

<Configuration Example 1 of Pixel>

The pixel 211 included in the imaging device 200 is formed with a plurality of subpixels 212, and each subpixel 212 is combined with a filter which transmits light in a specific wavelength range (color filter), whereby data for achieving color image display can be obtained.

FIG. 32A is a plan view showing an example of the pixel 211 with which a color image is obtained. The pixel 211 illustrated in FIG. 32A includes a subpixel 212 provided with a color filter transmitting light in a red (R) wavelength range (also referred to as a subpixel 212R), a subpixel 212 provided with a color filter transmitting light in a green (G) wavelength range (also referred to as a subpixel 212G), and a subpixel 212 provided with a color filter transmitting light in a blue (B) wavelength range (also referred to as a subpixel 212B). The subpixel 212 can function as a photosensor.

The subpixel 212 (the subpixels 212R, 212G, and 212B) is electrically connected to wirings 231, 247, 248, 249, and 250. In addition, the subpixels 212R, 212G, and 212B are connected to respective wirings 253 which are independent of one another. In this specification, for example, the wirings 248 and 249 that are connected to the pixel 211 in the n-th row (n is an integer greater than or equal to 1 and less than or equal top) are referred to as a wiring 248[n] and a wiring 249[n]. For example, the wiring 253 connected to the pixel 211 in the m-th column (in is an integer greater than or equal to 1 and less than or equal to q) is referred to as a wiring 253[m]. Note that in FIG. 32A, the wirings 253 connected to the subpixels 212R, 212G, and 212B in the pixel 211 in the m-th column are referred to as a wiring 253[m]R, a wiring 253[m]G, and a wiring 253[m]B. The subpixels 212 are electrically connected to the peripheral circuit through the above wirings.

The imaging device 200 has a structure in which the subpixel 212 is electrically connected to the subpixel 212 in an adjacent pixel 211 which is provided with a color filter transmitting light in the same wavelength range as the subpixel 212, via a switch. FIG. 32B shows a connection example of the subpixels 212: the subpixel 212 in the pixel 211 arranged in an n-th row and an m-th column and the subpixel 212 in the adjacent pixel 211 arranged in an (n+1)-th row and the m-th column. In FIG. 32B, the subpixel 212R arranged in the n-th row and the m-th column and the subpixel 212R arranged in the (n+1)-th row and the m-th column are connected to each other via a switch 201. The subpixel 212G arranged in the n-th row and the m-th column and the subpixel 212G arranged in the (n+1)-th row and the m-th column are connected to each other via a switch 202. The subpixel 212B arranged in the n-th row and the m-th column and the subpixel 212B arranged in the (n+1)-th row and the m-th column are connected to each other via a switch 203.

The color filter used in the subpixel 212 is not limited to red (R), green (G), and blue (B) color filters, and color filters that transmit light of cyan (C), yellow (Y), and magenta (M) may be used. By provision of the subpixels 212 that sense light in three different wavelength ranges in one pixel 211, a full-color image can be obtained.

The pixel 211 including the subpixel 212 provided with a color filter transmitting yellow (Y) light may be provided, in addition to the subpixels 212 provided with the color filters transmitting red (R), green (G), and blue (B) light. The pixel 211 including the subpixel 212 provided with a color filter transmitting blue (B) light may be provided, in addition to the subpixels 212 provided with the color filters transmitting cyan (C), yellow (Y), and magenta (M) light. When the subpixels 212 sensing light in four different wavelength ranges are provided in one pixel 211, the reproducibility of colors of an obtained image can be increased.

For example, in FIG. 32A, in regard to the subpixel 212 sensing light in a red wavelength range, the subpixel 212 sensing light in a green wavelength range, and the subpixel 212 sensing light in a blue wavelength range, the pixel number ratio (or the light receiving area ratio) thereof is not necessarily 1:1:1. For example, the Bayer arrangement in which the pixel number ratio (the light receiving area ratio) of red to green and blue is 1:2:1 may be employed. Alternatively, the pixel number ratio (the light receiving area ratio) of red to green and blue may be 1:6:1.

Although the number of subpixels 212 provided in the pixel 211 may be one, two or more subpixels are preferably provided. For example, when two or more subpixels 212 sensing light in the same wavelength range are provided, the redundancy is increased, and the reliability of the imaging device 200 can be increased.

When an infrared (IR) filter that transmits infrared light and absorbs or reflects visible light is used as the filter, the imaging device 200 that senses infrared light can be provided.

Furthermore, when a neutral density (ND) filter (dark filter) is used, output saturation which occurs when a large amount of light enters a photoelectric conversion element (light-receiving element) can be prevented. With a combination of ND filters with different dimming capabilities, the dynamic range of the imaging device can be increased.

Besides the above-described filter, the pixel 211 may be provided with a lens. An arrangement example of the pixel 211, a filter 254, and a lens 255 is described with cross-sectional views in FIGS. 33A and 33B. With the lens 255, the photoelectric conversion element can receive incident light efficiently. Specifically, as illustrated in FIG. 33A, light 256 enters a photoelectric conversion element 220 through the lens 255, the filter 254 (filters 254R, 254G, and 254B), a pixel circuit 230, and the like which are provided in the pixel 211.

However, as indicated by a region surrounded with dashed-dotted lines, part of the light 256 indicated by arrows might be blocked by some wirings 257. Thus, a preferable structure is that the lens 255 and the filter 254 are provided on the photoelectric conversion element 220 side, so that the photoelectric conversion element 220 can efficiently receive the light 256 as illustrated in FIG. 33B. When the light 256 enters the photoelectric conversion element 220 from the photoelectric conversion element 220 side, the imaging device 200 with high detection sensitivity can be provided.

As the photoelectric conversion element 220 illustrated in FIGS. 33A and 33B, a photoelectric conversion element in which a p-n junction or a p-i-n junction is formed may be used.

The photoelectric conversion element 220 may be formed using a substance that has a function of absorbing a radiation and generating electric charges. Examples of the substance that has a function of absorbing a radiation and generating electric charges include selenium, lead iodide, mercury iodide, gallium arsenide, cadmium telluride, and cadmium zinc alloy.

For example, when selenium is used for the photoelectric conversion element 220, the photoelectric conversion element 220 can have a light absorption coefficient in a wide wavelength range, such as visible light, ultraviolet light, infrared light, X-rays, and gamma rays.

One pixel 211 included in the imaging device 200 may include the subpixel 212 with a first filter in addition to the subpixel 212 illustrated in FIGS. 32A and 32B.

<Configuration Example 2 of Pixel>

An example of a pixel including a transistor using silicon and a transistor using an oxide semiconductor is described below.

FIGS. 34A and 34B are each a cross-sectional view of an element included in an imaging device.

The imaging device illustrated in FIG. 34A includes a transistor 351 including silicon over a silicon substrate 300, a transistor 353 which includes an oxide semiconductor and is stacked over the transistor 351, and a photodiode 360 provided in the silicon substrate 300 and including an anode 361 and a cathode 362. The transistors and the photodiode 360 are electrically connected to various plugs 370 and wirings 371, 372, and 373. In addition, the anode 361 of the photodiode 360 is electrically connected to the plug 370 through a low-resistance region 363.

The imaging device includes a layer 310 including the transistor 351 provided on the silicon substrate 300 and the photodiode 360 provided in the silicon substrate 300, a layer 320 which is in contact with the layer 310 and includes the wirings 371, a layer 330 which is in contact with the layer 320 and includes the transistor 353 and an insulating layer 380, and a layer 340 which is in contact with the layer 330 and includes the wiring 372 and the wiring 373.

Note that in the example of the cross-sectional view in FIG. 34A, a light-receiving surface of the photodiode 360 is provided on the side opposite to a surface of the silicon substrate 300 where the transistor 351 is formed. With the structure, an optical path can be obtained without the influence by the transistors, wirings, and the like. Thus, a pixel with a high aperture ratio can be formed. Note that the light-receiving surface of the photodiode 360 can be the same as the surface where the transistor 351 is formed.

In the case where a pixel is formed with the use of only transistors using an oxide semiconductor, the layer 310 may include the transistor using an oxide semiconductor. Alternatively, the layer 310 may be omitted, and the pixel may include only transistors using an oxide semiconductor.

In addition, in the cross-sectional view in FIG. 34A, the photodiode 360 in the layer 310 and the transistor in the layer 330 can be formed so as to overlap with each other. Thus, the degree of integration of pixels can be increased. In other words, the resolution of the imaging device can be increased.

An imaging device illustrated in FIG. 34B includes a photodiode 365 in the layer 340 and over the transistor. In FIG. 34B, the layer 310 includes the transistor 351 using silicon, the layer 320 includes the wiring 371, the layer 330 includes the transistor 353 using an oxide semiconductor and the insulating layer 380, and the layer 340 includes the photodiode 365. The photodiode 365 is electrically connected to the wiring 373 and a wiring 374 through the plug 370.

The element structure illustrated in FIG. 34B can increase the aperture ratio.

Alternatively, a PIN diode element formed using an amorphous silicon film, a microcrystalline silicon film, or the like may be used as the photodiode 365. In the photodiode 365, an n-type semiconductor 368, an i-type semiconductor 367, and a p-type semiconductor 366 are stacked in this order. The i-type semiconductor 367 is preferably formed using amorphous silicon. The p-type semiconductor 366 and the n-type semiconductor 368 can each be formed using amorphous silicon, microcrystalline silicon, or the like which includes a dopant imparting the corresponding conductivity type. The photodiode 365 in which a photoelectric conversion layer is formed using amorphous silicon has high sensitivity in a visible light wavelength region, and therefore can easily sense weak visible light.

Note that this embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

Embodiment 4

In this embodiment, circuit configuration examples to which the transistors including oxide semiconductor layers (OS transistors) described in the above embodiment can be applied will be described with reference to FIGS. 35A to 35C, FIGS. 36A to 36C, FIGS. 37A and 37B, and FIGS. 38A and 38B.

FIG. 35A is a circuit diagram of an inverter applicable to a memory, an FPGA, a CPU, or the like. An inverter 2800 outputs a signal whose logic is inverted from the logic of a signal supplied to an input terminal IN to an output terminal OUT. The inverter 2800 includes a plurality of OS transistors. A signal S_(BG) can switch electrical characteristics of the OS transistors.

FIG. 35B is a circuit diagram illustrating an example of the inverter 2800. The inverter 2800 includes OS transistors 2810 and 2820. The inverter 2800 can be formed using n-channel transistors and can have a circuit configuration in which all the transistors have the same conductivity. With the circuit configuration in which all the transistors have the same conductivity, the inverter can be formed at lower cost than an inverter formed using a complementary metal oxide semiconductor (i.e., a CMOS inverter).

Note that the inverter 2800 including the OS transistors can be provided over a CMOS circuit including Si transistors. Since the inverter 2800 can be provided so as to overlap with the CMOS circuit, no additional area is required for the inverter 2800, and thus, an increase in the circuit area can be suppressed.

Each of the OS transistors 2810 and 2820 includes a first gate functioning as a front gate, a second gate functioning as a back gate, a first terminal functioning as one of a source and a drain, and a second terminal functioning as the other of the source and the drain.

The first gate of the OS transistor 2810 is connected to its second terminal. The second gate of the OS transistor 2810 is connected to a wiring that transmits the signal S_(BG). The first terminal of the OS transistor 2810 is connected to a wiring that supplies a voltage VDD. The second terminal of the OS transistor 2810 is connected to the output terminal OUT.

The first gate of the OS transistor 2820 is connected to the input terminal IN. The second gate of the OS transistor 2820 is connected to the input terminal IN. The first terminal of the OS transistor 2820 is connected to the output terminal OUT. The second terminal of the OS transistor 2820 is connected to a wiring that supplies a voltage VSS.

FIG. 35C is a timing chart illustrating the operation of the inverter 2800. The timing chart in FIG. 35C illustrates changes of a signal waveform of the input terminal IN, a signal waveform of the output terminal OUT, a signal waveform of the signal S_(BG), and the threshold voltage of the OS transistor 2810.

The signal S_(BG) supplied to the second gate of the OS transistor 2810 can control the threshold voltage of the OS transistor 2810.

The signal S_(BG) includes a voltage V_(BG) _(_) _(A) for shifting the threshold voltage in the negative direction and a voltage V_(BG) _(_) _(B) for shifting the threshold voltage in the positive direction. The threshold voltage of the OS transistor 2810 can be shifted in the negative direction to be a threshold voltage V_(TH) _(_) _(A) when the voltage V_(BG) _(_) _(A) is applied to the second gate. The threshold voltage of the OS transistor 2810 can be shifted in the positive direction to be a threshold voltage V_(TH) _(_) _(B) when the voltage V_(BG) _(_) _(B) is applied to the second gate.

To visualize the above description, FIG. 36A shows a V_(g)-I_(d) curve, which is one of the electrical characteristics of a transistor.

When a high voltage such as the voltage V_(BG) _(_) _(A) is applied to the second gate, the electrical characteristics of the OS transistor 2810 can be shifted to match a curve shown by a dashed line 2840 in FIG. 36A. When a low voltage such as the voltage V_(BG) _(_) _(B) is applied to the second gate, the electrical characteristics of the OS transistor 2810 can be shifted to match a curve shown by a solid line 2841 in FIG. 36A. As shown in FIG. 36A, switching the signal S_(BG) between the voltage V_(BG) _(_) _(A) and the voltage V_(BG) _(_) _(B) enables the threshold voltage of the OS transistor 2810 to be shifted in the positive direction or the negative direction.

The shift of the threshold voltage in the positive direction toward the threshold voltage V_(TH) _(_) _(B) can make current less likely to flow in the OS transistor 2810. FIG. 36B visualizes the state. As illustrated in FIG. 36B, a current I_(B) that flows in the OS transistor 2810 can be extremely low. Thus, when a signal supplied to the input terminal IN is at a high level and the OS transistor 2820 is on (ON), the voltage of the output terminal OUT can drop sharply.

Since a state in which current is less likely to flow in the OS transistor 2810 as illustrated in FIG. 36B can be obtained, a signal waveform 2831 of the output terminal in the timing chart in FIG. 35C can be made steep. Shoot-through current between the wiring that supplies the voltage VDD and the wiring that supplies the voltage VSS can be low, leading to low-power operation.

The shift of the threshold voltage in the negative direction toward the threshold voltage V_(TH) _(_) _(A) can make current flow easily in the OS transistor 2810. FIG. 36C visualizes the state. As illustrated in FIG. 36C, a current I_(A) flowing at this time can be higher than at least the current I_(B). Thus, when a signal supplied to the input terminal IN is at a low level and the OS transistor 2820 is off (OFF), the voltage of the output terminal OUT can be increased sharply.

Since a state in which current is likely to flow in the OS transistor 2810 as illustrated in FIG. 36C can be obtained, a signal waveform 2832 of the output terminal in the timing chart in FIG. 35C can be made steep.

Note that the threshold voltage of the OS transistor 2810 is preferably controlled by the signal S_(BG) before the state of the OS transistor 2820 is switched, i.e., before Time T1 or T2. For example, as in FIG. 35C, it is preferable that the threshold voltage of the OS transistor 2810 be switched from the threshold voltage V_(TH) _(_) _(A) to the threshold voltage V_(TH) _(_) _(B) before Time T1 at which the level of the signal supplied to the input terminal IN is switched to a high level. Moreover, as in FIG. 35C, it is preferable that the threshold voltage of the OS transistor 2810 be switched from the threshold voltage V_(TH) _(_) _(B) to the threshold voltage V_(TH) _(_) _(A) before Time T2 at which the level of the signal supplied to the input terminal IN is switched to a low level.

Although the timing chart in FIG. 35C illustrates the configuration in which the level of the signal S_(BG) is switched in accordance with the signal supplied to the input terminal IN, a different configuration may be employed in which voltage for controlling the threshold voltage is held by the second gate of the OS transistor 2810 in a floating state, for example. FIG. 37A illustrates an example of such a circuit configuration.

The circuit configuration in FIG. 37A is the same as that in FIG. 35B, except that an OS transistor 2850 is added. A first terminal of the OS transistor 2850 is connected to the second gate of the OS transistor 2810. A second terminal of the OS transistor 2850 is connected to a wiring that supplies the voltage V_(BG) _(_) _(B) (or the voltage V_(BG) _(_) _(A)). A first gate of the OS transistor 2850 is connected to a wiring that supplies a signal S_(F). A second gate of the OS transistor 2850 is connected to the wiring that supplies the voltage V_(BG) _(_) _(B) (or the voltage V_(BG) _(_) _(A)).

The operation with the circuit configuration in FIG. 37A is described with reference to the timing chart in FIG. 37B.

The voltage for controlling the threshold voltage of the OS transistor 2810 is supplied to the second gate of the OS transistor 2810 before Time T3 at which the level of the signal supplied to the input terminal IN is switched to a high level. The signal S_(F) is set to a high level and the OS transistor 2850 is turned on, so that the voltage V_(BG) _(_) _(B) for controlling the threshold voltage is supplied to a node N_(BG).

The OS transistor 2850 is turned off after the voltage of the node N_(BG) becomes V_(BG) _(_) _(B). Since the off-state current of the OS transistor 2850 is extremely low, the voltage V_(BG) _(_) _(B) held by the node N_(BG) can be retained while the OS transistor 2850 remains off. Therefore, the number of times the voltage V_(BG) _(_) _(B) is supplied to the second gate of the OS transistor 2850 can be reduced and accordingly, the power consumption for rewriting the voltage V_(BG) _(_) _(B) can be reduced.

Although FIG. 35B and FIG. 37A each illustrate the case where the voltage is supplied to the second gate of the OS transistor 2810 by control from the outside, a different configuration may be employed in which voltage for controlling the threshold voltage is generated on the basis of the signal supplied to the input terminal IN and supplied to the second gate of the OS transistor 2810, for example. FIG. 38A illustrates an example of such a circuit configuration.

The circuit configuration in FIG. 38A is the same as that in FIG. 35B, except that a CMOS inverter 2860 is provided between the input terminal IN and the second gate of the OS transistor 2810. An input terminal of the CMOS inverter 2860 is connected to the input terminal IN. An output terminal of the CMOS inverter 2860 is connected to the second gate of the OS transistor 2810.

The operation with the circuit configuration in FIG. 38A is described with reference to a timing chart in FIG. 38B. The timing chart in FIG. 38B illustrates changes of a signal waveform of the input terminal IN, a signal waveform of the output terminal OUT, an output waveform IN_B of the CMOS inverter 2860, and a threshold voltage of the OS transistor 2810.

The output waveform IN_B which corresponds to a signal whose logic is inverted from the logic of the signal supplied to the input terminal IN can be used as a signal that controls the threshold voltage of the OS transistor 2810. Therefore, the threshold voltage of the OS transistor 2810 can be controlled as described with reference to FIGS. 36A to 36C. For example, the signal supplied to the input terminal IN is at a high level and the OS transistor 2820 is turned on at Time T4 in FIG. 38B. At this time, the output waveform IN_B is at a low level. Accordingly, current can be made less likely to flow in the OS transistor 2810; thus, a drop in the voltage of the output terminal OUT can be made steep.

Moreover, the signal supplied to the input terminal IN is at a low level and the OS transistor 2820 is turned off at Time T5 in FIG. 38B. At this time, the output waveform IN_B is at a high level. Accordingly, current can easily flow in the OS transistor 2810; thus, a rise in the voltage of the output terminal OUT can be made steep.

As described above, in the configuration of the inverter including the OS transistor in this embodiment, the voltage of the back gate is switched in accordance with the logic of the signal supplied to the input terminal IN. In such a configuration, the threshold voltage of the OS transistor can be controlled. The control of the threshold voltage of the OS transistor by the signal supplied to the input terminal IN can make a change in the voltage of the output terminal OUT steep. Moreover, shoot-through current between the wirings that supply power supply voltages can be reduced. Thus, power consumption can be reduced.

Embodiment 5

In this embodiment, examples of a semiconductor device which includes a plurality of circuits including OS transistors and described in the above embodiment are described with reference to FIGS. 39A to 39E, FIGS. 40A and 40B, FIGS. 41A and 41B, and FIGS. 42A to 42C.

<8. Circuit Configuration Example of Semiconductor Device>

FIG. 39A is a block diagram of a semiconductor device 900. The semiconductor device 900 includes a power supply circuit 901, a circuit 902, a voltage generation circuit 903, a circuit 904, a voltage generation circuit 905, and a circuit 906.

The power supply circuit 901 is a circuit that generates a voltage V_(ORG) used as a reference. The voltage V_(ORG) is not necessarily one voltage and can be a plurality of voltages. The voltage V_(ORG) can be generated on the basis of a voltage V₀ supplied from the outside of the semiconductor device 900. The semiconductor device 900 can generate the voltage V_(ORG) on the basis of one power supply voltage supplied from the outside. Thus, the semiconductor device 900 can operate without supply of a plurality of power supply voltages from the outside.

The circuits 902, 904, and 906 operate with different power supply voltages. For example, the power supply voltage of the circuit 902 is a voltage based on the voltage V_(ORG) and the voltage V_(SS) (V_(ORG)>V_(SS)), the power supply voltage of the circuit 904 is a voltage based on a voltage V_(POG) and the voltage V_(SS) (V_(POG)>V_(ORG)), and the power supply voltages of the circuit 906 are voltages based on the voltage V_(ORG), the voltage V_(SS), and a voltage V_(NEG) (V_(ORG)>V_(SS)>V_(NEG)). When the voltage V_(SS) is set to a ground potential (GND), the kinds of voltages generated by the power supply circuit 901 can be reduced.

The voltage generation circuit 903 is a circuit that generates the voltage V_(POG). The voltage generation circuit 903 can generate the voltage V_(POG) on the basis of the voltage V_(ORG) supplied from the power supply circuit 901. Thus, the semiconductor device 900 including the circuit 904 can operate on the basis of one power supply voltage supplied from the outside.

The voltage generation circuit 905 is a circuit that generates the voltage V_(NEG). The voltage generation circuit 905 can generate the voltage V_(NEG) on the basis of the voltage V_(ORG) supplied from the power supply circuit 901. Thus, the semiconductor device 900 including the circuit 906 can operate on the basis of one power supply voltage supplied from the outside.

FIG. 39B illustrates an example of the circuit 904 that operates with the voltage V_(POG) and FIG. 39C illustrates an example of a waveform of a signal for operating the circuit 904.

FIG. 39B illustrates a transistor 911. A signal supplied to a gate of the transistor 911 is generated on the basis of, for example, the voltage V_(POG) and the voltage V_(SS). The signal is generated on the basis of the voltage V_(POG) to turn on the transistor 911 and on the basis of the voltage V_(SS) to turn off the transistor 911. As illustrated in FIG. 39C, the voltage V_(POG) is higher than the voltage V_(ORG). Thus, a source (S) and a drain (D) of the transistor 911 can be electrically connected to each other without fail. As a result, the frequency of malfunction of the circuit 904 can be reduced.

FIG. 39D illustrates an example of the circuit 906 that operates with the voltage V_(NEG) and FIG. 39E illustrates an example of a waveform of a signal for operating the circuit 906.

FIG. 39D illustrates a transistor 912 having a back gate. A signal supplied to a gate of the transistor 912 is generated on the basis of, for example, the voltage V_(ORG) and the voltage V_(SS). The signal is generated on the basis of the voltage V_(ORG) to turn on the transistor 912 and on the basis of the voltage V_(SS) to turn off the transistor 912. A signal supplied to the back gate of the transistor 912 is generated on the basis of the voltage V_(NEG). As illustrated in FIG. 39E, the voltage V_(NEG) is lower than the voltage V_(SS) (GND). Thus, the threshold voltage of the transistor 912 can be controlled to shift in the positive direction. Thus, the transistor 912 can be turned off without fail and a current flowing between a source (S) and a drain (D) can be reduced. As a result, the frequency of malfunction of the circuit 906 can be reduced and power consumption thereof can be reduced.

The voltage V_(NEG) may be directly supplied to the back gate of the transistor 912. Alternatively, a signal supplied to the gate of the transistor 912 may be generated on the basis of the voltage V_(ORG) and the voltage V_(NEG) and the generated signal may also be supplied to the back gate of the transistor 912.

FIGS. 40A and 40B illustrate a modification example of FIGS. 39D and 39E.

In a circuit diagram illustrated in FIG. 40A, a transistor 922 whose on/off state can be controlled by a control circuit 921 is provided between the voltage generation circuit 905 and the circuit 906. The transistor 922 is an n-channel OS transistor. The control signal S_(BG) output from the control circuit 921 is a signal for controlling the on/off state of the transistor 922. Transistors 912A and 912B included in the circuit 906 are OS transistors like the transistor 922.

A timing chart in FIG. 40B shows changes in a potential of the control signal S_(BG) and a potential of a node N_(BG). The potential of the node N_(BG) indicates the states of potentials of back gates of the transistors 912A and 912B. When the control signal S_(BG) is at a high level, the transistor 922 is turned on and the voltage of the node N_(BG) becomes the voltage V_(NEG). Then, when the control signal S_(BG) is at a low level, the node N_(BG) is brought into an electrically floating state. Since the transistor 922 is an OS transistor, its off-state current is small. Accordingly, even when the node N_(BG) is in an electrically floating state, the voltage V_(NEG) which has been supplied can be held.

FIG. 41A illustrates an example of a circuit configuration applicable to the above-described voltage generation circuit 903. The voltage generation circuit 903 illustrated in FIG. 41A is a five-stage charge pump including diodes D1 to D5, capacitors C1 to C5, and an inverter INV. A clock signal CLK is supplied to the capacitors C1 to C5 directly or through the inverter INV. When a power supply voltage of the inverter INV is a voltage based on the voltage V_(ORG) and the voltage V_(SS), the voltage V_(POG) can be obtained by increasing the voltage V_(ORG) by a voltage five times a potential difference between the voltage V_(ORG) and the voltage V_(SS). Note that the forward voltage of the diodes D1 to D5 is 0 V. The number of stages of the charge pump can be changed to obtain a desired voltage V_(POG).

FIG. 41B illustrates an example of a circuit configuration applicable to the above-described voltage generation circuit 905. The voltage generation circuit 905 illustrated in FIG. 41B is a four-stage charge pump including the diodes D1 to D5, the capacitors C1 to C5, and the inverter INV. The clock signal CLK is supplied to the capacitors C1 to C5 directly or through the inverter INV. When a power supply voltage of the inverter INV is a voltage applied on the basis of the voltage V_(ORG) and the voltage V_(SS), the voltage V_(NEG) can be obtained by decreasing the ground voltage, i.e., the voltage V_(SS) by a voltage four times the potential difference between the voltage V_(ORG) and the voltage V_(SS) with the application of the clock signal CLK. Note that the forward voltage of the diodes D1 to D5 is 0 V. The number of stages of the charge pump can be changed to obtain a desired voltage V_(NEG).

The circuit configuration of the voltage generation circuit 903 is not limited to the configuration in the circuit diagram illustrated in FIG. 41A. Modification examples of the voltage generation circuit 903 are illustrated in FIGS. 42A to 42C. Note that further modification examples of the voltage generation circuit 903 can be realized by changing voltages supplied to wirings or arrangement of elements in voltage generation circuits 903A to 903C illustrated in FIGS. 42A to 42C

The voltage generation circuit 903A illustrated in FIG. 42A includes transistors M1 to M10, capacitors C11 to C14, and an inverter INV1. The clock signal CLK is supplied to gates of the transistors M1 to M10 directly or through the inverter INV1. By application of the clock signal CLK, the voltage V_(POG), which has been increased to a positive voltage having a positively quadrupled value of the voltage V_(ORG), can be obtained. The number of stages can be changed to obtain a desired voltage V_(POG). In the voltage generation circuit 903A in FIG. 42A, the off-state current of each of the transistors M1 to M10 can be small when the transistors M1 to M10 are OS transistors, and the leakage of charge held in the capacitors C11 to C14 can be reduced. Accordingly, the voltage V_(ORG) can be efficiently increased to the voltage V_(POG).

The voltage generation circuit 903B illustrated in FIG. 42B includes transistors M11 to M14, capacitors C15 and C16, and an inverter INV2. The clock signal CLK is supplied to gates of the transistors M11 to M14 directly or through the inverter INV2. By application of the clock signal CLK, the voltage V_(POG), which has been increased to a positive voltage having a positively doubled value of the voltage V_(ORG), can be obtained. In the voltage generation circuit 903B in FIG. 42B, off-state current of each of the transistors M11 to M14 can be small when the transistors M11 to M14 are OS transistors, and leakage of charge held in the capacitors C15 and C16 can be suppressed. Accordingly, the voltage V_(ORG) can be efficiently increased to the voltage V_(POG).

A voltage generation circuit 903C illustrated in FIG. 42C includes an inductor I1, a transistor M15, a diode D6, and a capacitor C17. The on/off state of the transistor M15 is controlled by a control signal EN. Owing to the control signal EN, the voltage V_(POG) increased from the voltage V_(ORG) can be obtained. Since the voltage generation circuit 903C in FIG. 42C increases the voltage using the inductor I1, the voltage can be efficiently increased.

As described above, in any of the structures of this embodiment, a voltage required for circuits included in a semiconductor device can be internally generated. Thus, in the semiconductor device, the number of power supply voltages supplied from the outside can be reduced.

Embodiment 6

<RF Tag>

In this embodiment, an RF tag that includes the transistor described in the above embodiment or the memory device described in the above embodiment will be described with reference to FIG. 43.

The RF tag of this embodiment includes a memory circuit, stores necessary data in the memory circuit, and transmits and receives data to/from the outside by using a contactless means, for example, wireless communication. With these features, the RF tag can be used for an individual authentication system in which an object or the like is recognized by reading the individual information, for example. Note that the RF tag is required to have extremely high reliability in order to be used for this purpose.

A configuration of the RF tag is described with reference to FIG. 43. FIG. 43 is a block diagram illustrating a configuration example of an RF tag.

As shown in FIG. 43, an RF tag 800 includes an antenna 804 which receives a radio signal 803 that is transmitted from an antenna 802 connected to a communication device 801 (also referred to as an interrogator, a reader/writer, or the like). The RF tag 800 includes a rectifier circuit 805, a constant voltage circuit 806, a demodulation circuit 807, a modulation circuit 808, a logic circuit 809, a memory circuit 810, and a ROM 811. A transistor having a rectifying function included in the demodulation circuit 807 may be formed using a material which enables a reverse current to be low enough, for example, an oxide semiconductor. This can suppress the phenomenon of a rectifying function becoming weaker due to generation of a reverse current and prevent saturation of the output from the demodulation circuit. In other words, the input to the demodulation circuit and the output from the demodulation circuit can have a relation closer to a linear relation. Note that data transmission methods are roughly classified into the following three methods: an electromagnetic coupling method in which a pair of coils is provided so as to face each other and communicates with each other by mutual induction, an electromagnetic induction method in which communication is performed using an induction field, and a radio wave method in which communication is performed using a radio wave. Any of these methods can be used in the RF tag 800 described in this embodiment.

Next, the structure of each circuit is described. The antenna 804 exchanges the radio signal 803 with the antenna 802 which is connected to the communication device 801. The rectifier circuit 805 generates an input potential by rectification, for example, half-wave voltage doubler rectification of an input alternating signal generated by reception of a radio signal at the antenna 804 and smoothing of the rectified signal with a capacitor provided in a later stage. Note that a limiter circuit may be provided on an input side or an output side of the rectifier circuit 805. The limiter circuit controls electric power so that electric power which is higher than or equal to certain electric power is not input to a circuit in a later stage if the amplitude of the input alternating signal is high and an internal generation voltage is high.

The constant voltage circuit 806 generates a stable power supply voltage from an input potential and supplies it to each circuit. Note that the constant voltage circuit 806 may include a reset signal generation circuit. The reset signal generation circuit is a circuit which generates a reset signal of the logic circuit 809 by utilizing rise of the stable power supply voltage.

The demodulation circuit 807 demodulates the input alternating signal by envelope detection and generates the demodulated signal. Further, the modulation circuit 808 performs modulation in accordance with data to be output from the antenna 804.

The logic circuit 809 analyzes and processes the demodulated signal. The memory circuit 810 holds the input data and includes a row decoder, a column decoder, a memory region, and the like. Further, the ROM 811 stores an identification number (ID) or the like and outputs it in accordance with processing.

Note that the decision whether each circuit described above is provided or not can be made as appropriate as needed.

Here, the memory circuit described in the above embodiment can be used for the memory circuit 810. Since the memory circuit of one embodiment of the present invention can retain data even when not powered, the memory circuit can be favorably used for an RF tag. Furthermore, the memory circuit of one embodiment of the present invention needs power (voltage) needed for data writing significantly lower than that needed in a conventional nonvolatile memory; thus, it is possible to prevent a difference between the maximum communication range in data reading and that in data writing. In addition, it is possible to suppress malfunction or incorrect writing which is caused by power shortage in data writing.

Since the memory circuit of one embodiment of the present invention can be used as a nonvolatile memory, it can also be used as the ROM 811. In this case, it is preferable that a manufacturer separately prepare a command for writing data to the ROM 811 so that a user cannot rewrite data freely. Since the manufacturer gives identification numbers before shipment and then starts shipment of products, instead of putting identification numbers to all the manufactured RF tags, it is possible to put identification numbers to only good products to be shipped. Thus, the identification numbers of the shipped products are in series and customer management corresponding to the shipped products is easily performed.

Note that this embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

Embodiment 7

In this embodiment, a CPU that includes the memory device described in the above embodiment will be described.

FIG. 44 is a block diagram illustrating a configuration example of a CPU at least partly including the transistor described in the above embodiment as a component.

<Circuit Diagram of CPU>

The CPU illustrated in FIG. 44 includes, over a substrate 1190, an arithmetic logic unit (ALU) 1191, an ALU controller 1192, an instruction decoder 1193, an interrupt controller 1194, a timing controller 1195, a register 1196, a register controller 1197, a bus interface 1198, a rewritable ROM 1199, and a ROM interface 1189. A semiconductor substrate, an SOI substrate, a glass substrate, or the like is used as the substrate 1190. The ROM 1199 and the ROM interface 1189 may be provided over a separate chip. Needless to say, the CPU in FIG. 44 is just an example in which the configuration is simplified, and an actual CPU may have a variety of configurations depending on the application. For example, the CPU may have the following configuration: a structure including the CPU in FIG. 44 or an arithmetic circuit is considered as one core; a plurality of the cores are included; and the cores operate in parallel. The number of bits that the CPU can process in an internal arithmetic circuit or in a data bus can be 8, 16, 32, or 64, for example.

An instruction that is input to the CPU through the bus interface 1198 is input to the instruction decoder 1193 and decoded therein, and then, input to the ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195 conduct various controls in accordance with the decoded instruction. Specifically, the ALU controller 1192 generates signals for controlling the operation of the ALU 1191. While the CPU is executing a program, the interrupt controller 1194 judges an interrupt request from an external input/output device or a peripheral circuit on the basis of its priority or a mask state, and processes the request. The register controller 1197 generates an address of the register 1196, and reads/writes data from/to the register 1196 in accordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operation timings of the ALU 1191, the ALU controller 1192, the instruction decoder 1193, the interrupt controller 1194, and the register controller 1197. For example, the timing controller 1195 includes an internal clock generator for generating an internal clock signal CLK2 based on a reference clock signal CLK1, and supplies the internal clock signal CLK2 to the above circuits.

In the CPU illustrated in FIG. 44, a memory cell is provided in the register 1196. For the memory cell of the register 1196, the transistor described in Embodiment 1 can be used.

In the CPU illustrated in FIG. 44, the register controller 1197 selects operation of retaining data in the register 1196 in accordance with an instruction from the ALU 1191. That is, the register controller 1197 selects whether data is retained by a flip-flop or by a capacitor in the memory cell included in the register 1196. When data retention by the flip-flop is selected, a power supply voltage is supplied to the memory cell in the register 1196. When data retention by the capacitor is selected, the data is rewritten in the capacitor, and supply of power supply voltage to the memory cell in the register 1196 can be stopped.

<Memory Circuit>

FIG. 45 is an example of a circuit diagram of a memory element that can be used as the register 1196. A memory element 1200 includes a circuit 1201 in which stored data is volatile when power supply is stopped, a circuit 1202 in which stored data is nonvolatile even when power supply is stopped, switches 1203 and 1204, a logic element 1206, a capacitor 1207, and a circuit 1220 having a selecting function. The circuit 1202 includes a capacitor 1208 and transistors 1209 and 1210. Note that the memory element 1200 may further include another element such as a diode, a resistor, or an inductor, as needed.

Here, the memory device described in the above embodiment can be used as the circuit 1202. When supply of a power supply voltage to the memory element 1200 is stopped, a ground potential (0 V) or a potential at which the transistor 1209 in the circuit 1202 is turned off continues to be input to a gate of the transistor 1209. For example, a first gate of the transistor 1209 is grounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213 having one conductivity type (e.g., an n-channel transistor) and the switch 1204 is a transistor 1214 having a conductivity type opposite to the one conductivity type (e.g., a p-channel transistor). A first terminal of the switch 1203 corresponds to one of a source and a drain of the transistor 1213, a second terminal of the switch 1203 corresponds to the other of the source and the drain of the transistor 1213, and conduction or non-conduction between the first terminal and the second terminal of the switch 1203 (i.e., the on/off state of the transistor 1213) is selected by a control signal RD input to a gate of the transistor 1213. A first terminal of the switch 1204 corresponds to one of a source and a drain of the transistor 1214, a second terminal of the switch 1204 corresponds to the other of the source and the drain of the transistor 1214, and conduction or non-conduction between the first terminal and the second terminal of the switch 1204 (i.e., the on/off state of the transistor 1214) is selected by the control signal RD input to a gate of the transistor 1214.

One of a source and a drain of the transistor 1209 is electrically connected to one of a pair of electrodes of the capacitor 1208 and a gate of the transistor 1210. Here, the connection portion is referred to as a node M2. One of a source and a drain of the transistor 1210 is electrically connected to a line which can supply a low power supply potential (e.g., a GND line), and the other thereof is electrically connected to the first terminal of the switch 1203 (the one of the source and the drain of the transistor 1213). The second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) is electrically connected to the first terminal of the switch 1204 (the one of the source and the drain of the transistor 1214). The second terminal of the switch 1204 (the other of the source and the drain of the transistor 1214) is electrically connected to a line which can supply a power supply potential VDD. The second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213), the first terminal of the switch 1204 (the one of the source and the drain of the transistor 1214), an input terminal of the logic element 1206, and one of a pair of electrodes of the capacitor 1207 are electrically connected to each other. Here, the connection portion is referred to as a node M1. The other of the pair of electrodes of the capacitor 1207 can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor 1207 can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor 1207 is electrically connected to the line which can supply a low power supply potential (e.g., a GND line). The other of the pair of electrodes of the capacitor 1208 can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor 1208 can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor 1208 is electrically connected to the line which can supply a low power supply potential (e.g., a GND line).

The capacitors 1207 and 1208 are not necessarily provided as long as the parasitic capacitance of the transistor, the wiring, or the like is actively utilized.

A control signal WE is input to the first gate (first gate electrode) of the transistor 1209. As for each of the switch 1203 and the switch 1204, a conduction state or a non-conduction state between the first terminal and the second terminal is selected by the control signal RD which is different from the control signal WE. When the first terminal and the second terminal of one of the switches are in the conduction state, the first terminal and the second terminal of the other of the switches are in the non-conduction state.

Note that the transistor 1209 in FIG. 45 has a structure with a second gate (second gate electrode: back gate). The control signal WE can be input to the first gate and a control signal WE2 can be input to the second gate. The control signal WE2 is a signal having a constant potential. As the constant potential, for example, a ground potential GND or a potential lower than a source potential of the transistor 1209 is selected. The control signal WE2 is a potential signal for controlling the threshold voltage of the transistor 1209, and a current when a gate voltage VG is 0 V can be further reduced. The control signal WE2 may be a signal having the same potential as the control signal WE. Note that as the transistor 1209, a transistor without a second gate may be used.

A signal corresponding to data retained in the circuit 1201 is input to the other of the source and the drain of the transistor 1209. FIG. 44 illustrates an example in which a signal output from the circuit 1201 is input to the other of the source and the drain of the transistor 1209. The logic value of a signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) is inverted by the logic element 1206, and the inverted signal is input to the circuit 1201 through the circuit 1220.

In the example of FIG. 45, a signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) is input to the circuit 1201 through the logic element 1206 and the circuit 1220; however, one embodiment of the present invention is not limited thereto. The signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) may be input to the circuit 1201 without its logic value being inverted. For example, in the case where the circuit 1201 includes a node in which a signal obtained by inversion of the logic value of a signal input from the input terminal is retained, the signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) can be input to the node.

In FIG. 45, the transistors included in the memory element 1200 except for the transistor 1209 can each be a transistor in which a channel is formed in a layer formed using a semiconductor other than an oxide semiconductor or in the substrate 1190. For example, the transistor can be a transistor whose channel is formed in a silicon layer or a silicon substrate. Alternatively, all the transistors in the memory element 1200 may be a transistor in which a channel is formed in an oxide semiconductor. Further alternatively, in the memory element 1200, a transistor in which a channel is formed in an oxide semiconductor can be included besides the transistor 1209, and a transistor in which a channel is formed in a layer including a semiconductor other than an oxide semiconductor or the substrate 1190 can be used for the rest of the transistors.

As the circuit 1201 in FIG. 45, for example, a flip-flop circuit can be used. As the logic element 1206, for example, an inverter or a clocked inverter can be used.

In a period during which the memory element 1200 is not supplied with the power supply voltage, the semiconductor device of one embodiment of the present invention can retain data stored in the circuit 1201 by the capacitor 1208 which is provided in the circuit 1202.

The off-state current of a transistor in which a channel is formed in an oxide semiconductor is extremely low. For example, the off-state current of a transistor in which a channel is formed in an oxide semiconductor is significantly lower than that of a transistor in which a channel is formed in silicon having crystallinity. Thus, when the transistor is used as the transistor 1209, a signal held in the capacitor 1208 is retained for a long time also in a period during which the power supply voltage is not supplied to the memory element 1200. The memory element 1200 can accordingly retain the stored content (data) also in a period during which the supply of the power supply voltage is stopped.

Since the above-described memory element performs pre-charge operation with the switches 1203 and 1204, it is possible to shorten the time required for the circuit 1201 to retain original data again after the supply of the power supply voltage is restarted.

In the circuit 1202, a signal retained by the capacitor 1208 is input to the gate of the transistor 1210. Therefore, after supply of the power supply voltage to the memory element 1200 is restarted, the state (the on state or the off state) of the transistor 1210 is determined in accordance with the signal retained by the capacitor 1208 and can be read from the circuit 1202. Consequently, an original signal can be accurately read even when a potential corresponding to the signal retained by the capacitor 1208 varies to some degree.

By applying the above-described memory element 1200 to a memory device such as a register or a cache memory included in a processor, data in the memory device can be prevented from being lost owing to the stop of the supply of the power supply voltage. Furthermore, shortly after the supply of the power supply voltage is restarted, the memory device can be returned to the same state as that before the power supply is stopped. Therefore, the power supply can be stopped even for a short time in the processor or one or a plurality of logic circuits included in the processor, resulting in lower power consumption.

Although the memory element 1200 is used in a CPU in this embodiment, the memory element 1200 can also be used in an LSI such as a digital signal processor (DSP), a custom LSI, or a programmable logic device (PLD), and a radio frequency (RF) tag.

Note that this embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

Embodiment 8

In this embodiment, configuration examples of a display device using a transistor of one embodiment of the present invention will be described.

<Circuit Configuration Example of Display Device>

FIG. 46A is a top view of the display device of one embodiment of the present invention. FIG. 46B is a circuit diagram illustrating a pixel circuit that can be used in the case where a liquid crystal element is used in a pixel in the display device of one embodiment of the present invention. FIG. 46C is a circuit diagram illustrating a pixel circuit that can be used in the case where an organic EL element is used in a pixel in the display device of one embodiment of the present invention.

The transistor in the pixel portion can be formed in accordance with Embodiment 1. The transistor can be easily formed as an n-channel transistor, and thus part of a driver circuit that can be formed using an n-channel transistor can be formed over the same substrate as the transistor of the pixel portion. With the use of the transistor described in the above embodiment for the pixel portion or the driver circuit in this manner, a highly reliable display device can be provided.

FIG. 46A illustrates an example of a top view of an active matrix display device. A pixel portion 701, a first scan line driver circuit 702, a second scan line driver circuit 703, and a signal line driver circuit 704 are formed over a substrate 700 of the display device. In the pixel portion 701, a plurality of signal lines extended from the signal line driver circuit 704 are arranged and a plurality of scan lines extended from the first scan line driver circuit 702 and the second scan line driver circuit 703 are arranged. Note that pixels which include display elements are provided in a matrix in respective regions where the scan lines and the signal lines intersect with each other. The substrate 700 of the display device is connected to a timing control circuit (also referred to as a controller or a controller IC) through a connection portion such as a flexible printed circuit (FPC).

In FIG. 46A, the first scan line driver circuit 702, the second scan line driver circuit 703, and the signal line driver circuit 704 are formed over the substrate 700 where the pixel portion 701 is formed. Accordingly, the number of components which are provided outside, such as a driver circuit, can be reduced, so that a reduction in cost can be achieved. Furthermore, if the driver circuit is provided outside the substrate 700, wirings would need to be extended and the number of wiring connections would increase. When the driver circuit is provided over the substrate 700, the number of wiring connections can be reduced. Consequently, an improvement in reliability or yield can be achieved. One or more of the first scan line driver circuit 702, the second scan line driver circuit 703, and the signal line driver circuit 704 may be mounted on the substrate 700 or provided outside the substrate 700.

<Liquid Crystal Display Device>

FIG. 46B illustrates an example of a circuit configuration of the pixel. Here, a pixel circuit which is applicable to a pixel of a VA liquid crystal display device is illustrated as an example.

This pixel circuit can be applied to a structure in which one pixel includes a plurality of pixel electrode layers. The pixel electrode layers are connected to different transistors, and the transistors can be driven with different gate signals. Accordingly, signals applied to individual pixel electrode layers in a multi-domain pixel can be controlled independently.

A scan line 712 of a transistor 716 and a scan line 713 of a transistor 717 are separated so that different gate signals can be supplied thereto. In contrast, a signal line 714 is shared by the transistors 716 and 717. The transistor described in Embodiment 1 can be used as appropriate as each of the transistors 716 and 717. Thus, a highly reliable liquid crystal display device can be provided.

A first pixel electrode layer is electrically connected to the transistor 716 and a second pixel electrode layer is electrically connected to the transistor 717. The first pixel electrode layer and the second pixel electrode layer are separated. There is no particular limitation on the shapes of the first pixel electrode layer and the second pixel electrode layer. For example, the first pixel electrode layer may have a V-like shape.

A gate electrode of the transistor 716 is connected to the scan line 712, and a gate electrode of the transistor 717 is connected to the scan line 713. When different gate signals are supplied to the scan line 712 and the scan line 713, operation timings of the transistors 716 and 717 can be varied. As a result, alignment of liquid crystals can be controlled.

Further, a storage capacitor may be formed using a capacitor wiring 710, a gate insulating layer functioning as a dielectric, and a capacitor electrode electrically connected to the first pixel electrode layer or the second pixel electrode layer.

The multi-domain pixel includes a first liquid crystal element 718 and a second liquid crystal element 719. The first liquid crystal element 718 includes the first pixel electrode layer, a counter electrode layer, and a liquid crystal layer therebetween. The second liquid crystal element 719 includes the second pixel electrode layer, a counter electrode layer, and a liquid crystal layer therebetween.

Note that a pixel circuit of the present invention is not limited to that illustrated in FIG. 46B. For example, a switch, a resistor, a capacitor, a transistor, a sensor, a logic circuit, or the like may be added to the pixel circuit illustrated in FIG. 46B.

FIGS. 47A and 47B are examples of a top view and a cross-sectional view of a liquid crystal display device. Note that FIG. 47A illustrates a typical structure including a display device 20, a display region 21, a peripheral circuit 22, and flexible printed circuits (FPCs) 42. The display device illustrated in FIGS. 47A and 47B uses a reflective liquid crystal element.

FIG. 47B is a cross-sectional view taken along dashed lines A-A′, B-B′, C-C′, and D-D′ in FIG. 47A. The cross section taken along dashed line A-A′ illustrates the peripheral circuit portion, the cross section taken along dashed line B-B′ illustrates the display region, and the cross sections taken along dashed line C-C′ and dashed line D-D′ illustrate portions connected to the FPCs.

The display device 20 using the liquid crystal element includes the following in addition to transistors 50 and 52 (the transistor 10 described in Embodiment 1): the conductive layer 165, a conductive layer 197, an insulating layer 420, a liquid crystal layer 490, a liquid crystal element 80, capacitors 60 and 62, an insulating layer 430, a spacer 440, a coloring layer 460, a bonding layer 470, a conductive layer 480, a light-shielding layer 418, a substrate 400, bonding layers 473, 474, 475, and 476, polarizing plates 103 and 403, protective substrates 105 and 402, and an anisotropic conductive layer 510.

<Organic EL Display Device>

FIG. 46C illustrates another example of a circuit configuration of the pixel. Here, a pixel structure of a display device using an organic EL element is shown.

In an organic EL element, by application of voltage to a light-emitting element, electrons are injected from one of a pair of electrodes and holes are injected from the other of the pair of electrodes, into a layer containing a light-emitting organic compound; thus, current flows. The electrons and holes are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Owing to such a mechanism, this light-emitting element is referred to as a current-excitation light-emitting element.

FIG. 46C illustrates an applicable example of a pixel circuit. Here, one pixel includes two n-channel transistors. Furthermore, digital time grayscale driving can be employed for the pixel circuit.

The configuration of the applicable pixel circuit and operation of a pixel employing digital time grayscale driving are described.

A pixel 720 includes a switching transistor 721, a driver transistor 722, a light-emitting element 724, and a capacitor 723. A gate electrode layer of the switching transistor 721 is connected to a scan line 726, a first electrode (one of a source electrode layer and a drain electrode layer) of the switching transistor 721 is connected to a signal line 725, and a second electrode (the other of the source electrode layer and the drain electrode layer) of the switching transistor 721 is connected to a gate electrode layer of the driver transistor 722. The gate electrode layer of the driver transistor 722 is connected to a power supply line 727 through the capacitor 723, a first electrode of the driver transistor 722 is connected to the power supply line 727, and a second electrode of the driver transistor 722 is connected to a first electrode (a pixel electrode) of the light-emitting element 724. A second electrode of the light-emitting element 724 corresponds to a common electrode 728. The common electrode 728 is electrically connected to a common potential line formed over the same substrate as the common electrode 728.

As the switching transistor 721 and the driver transistor 722, the transistor described in Embodiment 1 can be used as appropriate. In this manner, a highly reliable organic EL display device can be provided.

The potential of the second electrode (the common electrode 728) of the light-emitting element 724 is set to be a low power supply potential. Note that the low power supply potential is lower than a high power supply potential supplied to the power supply line 727. For example, the low power supply potential can be GND, 0 V, or the like. The high power supply potential and the low power supply potential are set to be higher than or equal to the forward threshold voltage of the light-emitting element 724, and the difference between the potentials is applied to the light-emitting element 724, whereby current is supplied to the light-emitting element 724, leading to light emission. The forward voltage of the light-emitting element 724 refers to a voltage at which a desired luminance is obtained, and includes at least a forward threshold voltage.

Note that gate capacitance of the driver transistor 722 may be used as a substitute for the capacitor 723, so that the capacitor 723 can be omitted.

Next, a signal input to the driver transistor 722 is described. In the case of a voltage-input voltage driving method, a video signal for sufficiently turning on or off the driver transistor 722 is input to the driver transistor 722. In order for the driver transistor 722 to operate in a linear region, voltage higher than the voltage of the power supply line 727 is applied to the gate electrode layer of the driver transistor 722. Note that voltage higher than or equal to voltage which is the sum of power supply line voltage and the threshold voltage V_(th) of the driver transistor 722 is applied to the signal line 725.

In the case of performing analog grayscale driving, a voltage higher than or equal to a voltage which is the sum of the forward voltage of the light-emitting element 724 and the threshold voltage V_(th) of the driver transistor 722 is applied to the gate electrode layer of the driver transistor 722. A video signal by which the driver transistor 722 is operated in a saturation region is input, so that current is supplied to the light-emitting element 724. In order for the driver transistor 722 to operate in a saturation region, the potential of the power supply line 727 is set higher than the gate potential of the driver transistor 722. When an analog video signal is used, it is possible to supply current to the light-emitting element 724 in accordance with the video signal and perform analog grayscale driving.

Note that the configuration of the pixel circuit of the present invention is not limited to that illustrated in FIG. 46C. For example, a switch, a resistor, a capacitor, a sensor, a transistor, a logic circuit, or the like may be added to the pixel circuit illustrated in FIG. 46C.

In the case where the transistor described in the above embodiment is used for the circuit illustrated in FIGS. 46A to 46C, the source electrode (the first electrode) is electrically connected to the low potential side and the drain electrode (the second electrode) is electrically connected to the high potential side. Furthermore, the potential of the first gate electrode may be controlled by a control circuit or the like, and the potential described above as an example, for example, a potential lower than the potential applied to the source electrode, may be input to the second gate electrode through a wiring that is not illustrated.

FIGS. 48A and 48B are examples of a top view and a cross-sectional view of a display device using a light-emitting element. Note that FIG. 48A illustrates a typical structure including a display device 24, the display region 21, the peripheral circuit 22, and the flexible printed circuit (FPCs) 42.

FIG. 48B is a cross-sectional view taken along dashed lines A-A′, B-B′, and C-C′ in FIG. 48A. The cross section taken along dashed line A-A′ illustrates the peripheral circuit portion, the cross section taken along dashed line B-B′ illustrates the display region, and the cross sections taken along dashed line C-C′ and dashed line D-D′ illustrate portions connected to the FPCs.

The display device 24 using the light-emitting element includes the following in addition to the transistors 50 and 52 (the transistor 10 described in Embodiment 1): the insulating layer 420, the conductive layer 197, a conductive layer 410, an optical adjustment layer 530, an EL layer 450, a conductive layer 415, a light-emitting element 70, the capacitors 60 and 62, the insulating layer 430, the spacer 440, the coloring layer 460, the bonding layer 470, a partition wall 445, the light-shielding layer 418, the substrate 400, and the anisotropic conductive layer 510.

In this specification, a display element, a display device which is a device including a display element, a light-emitting element, and a light-emitting device which is a device including a light-emitting element can employ a variety of modes or can include a variety of elements, for example. A display element, a display device, a light-emitting element, or a light-emitting device include at least one of the following, for example: an EL (electroluminescent) element (e.g., an EL element including organic and inorganic materials, an organic EL element, or an inorganic EL element), an LED (e.g., a white LED, a red LED, a green LED, or a blue LED), a quantum dot, a transistor (a transistor which emits light depending on current), an electron emitter, a liquid crystal element, electronic ink, an electrophoretic element, a grating light valve (GLV), a plasma display panel (PDP), micro electro mechanical systems (MEMS), a digital micromirror device (DMD), a digital micro shutter (DMS), MIRASOL (registered trademark), an interferometric modulator (IMOD) element, an electrowetting element, a piezoelectric ceramic display, and a display element using a carbon nanotube. Other than the above, display media whose contrast, luminance, reflectivity, transmittance, or the like is changed by electric or electromagnetic action may be included. Note that examples of display devices having EL elements include an EL display. Examples of display devices including electron emitters include a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). Examples of display devices including liquid crystal elements include a liquid crystal display (e.g., a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). Examples of display devices including electronic ink or electrophoretic elements include electronic paper.

Note that this embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

Embodiment 9

In this embodiment, a display module using a semiconductor device of one embodiment of the present invention will be described with reference to FIG. 49.

<Display Module>

In a display module 6000 in FIG. 49, a touch panel 6004 connected to an FPC 6003, a display panel 6006 connected to an FPC 6005, a backlight unit 6007, a frame 6009, a printed circuit board 6010, and a battery 6011 are provided between an upper cover 6001 and a lower cover 6002. Note that the backlight unit 6007, the battery 6011, the touch panel 6004, and the like are not provided in some cases.

The semiconductor device of one embodiment of the present invention can be used for, for example, the display panel 6006 and an integrated circuit mounted on a printed circuit board.

The shapes and sizes of the upper cover 6001 and the lower cover 6002 can be changed as appropriate in accordance with the sizes of the touch panel 6004 and the display panel 6006.

The touch panel 6004 can be a resistive touch panel or a capacitive touch panel and may be formed to overlap with the display panel 6006. A counter substrate (sealing substrate) of the display panel 6006 can have a touch panel function. A photosensor may be provided in each pixel of the display panel 6006 so that an optical touch panel function is added. An electrode for a touch sensor may be provided in each pixel of the display panel 6006 so that a capacitive touch panel function is added.

The backlight unit 6007 includes a light source 6008. The light source 6008 may be provided at an end portion of the backlight unit 6007 and a light diffusing plate may be used.

The frame 6009 protects the display panel 6006 and also functions as an electromagnetic shield for blocking electromagnetic waves generated from the printed circuit board 6010. The frame 6009 may function as a radiator plate.

The printed circuit board 6010 has a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or the battery 6011 provided separately may be used. Note that the battery 6011 is not necessary in the case where a commercial power source is used.

The display module 6000 can be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.

Note that this embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

Embodiment 10

In this embodiment, application examples of the semiconductor device of one embodiment of the present invention will be described.

<Package Using Lead Frame Interposer>

FIG. 50A is a perspective view illustrating a cross-sectional structure of a package using a lead frame interposer. In the package illustrated in FIG. 50A, a chip 1751 corresponding to the semiconductor device of one embodiment of the present invention is connected to a terminal 1752 over an interposer 1750 by wire bonding. The terminal 1752 is placed on a surface of the interposer 1750 on which the chip 1751 is mounted. The chip 1751 may be sealed by a mold resin 1753, in which case the chip 1751 is sealed such that part of each of the terminals 1752 is exposed.

FIG. 50B illustrates the structure of a module of an electronic device (mobile phone) in which a package is mounted on a circuit board. In the module of the mobile phone in FIG. 50B, a package 1802 and a battery 1804 are mounted on a printed wiring board 1801. The printed wiring board 1801 is mounted on a panel 1800 including a display element by an FPC 1803.

Note that this embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

Embodiment 11

In this embodiment, electronic devices and lighting devices of embodiments of the present invention will be described with reference to drawings.

<Electronic Device>

Electronic devices and lighting devices can be manufactured using the semiconductor device of one embodiment of the present invention. In addition, highly reliable electronic devices and lighting devices can be manufactured using the semiconductor device of one embodiment of the present invention. Furthermore, electronic devices and lighting devices including touch sensors with improved detection sensitivity can be fabricated using the semiconductor device of one embodiment of the present invention.

Examples of electronic devices are television devices (also referred to as TVs or television receivers), monitors for computers and the like, cameras such as digital cameras and digital video cameras, digital photo frames, cellular phones (also referred to as mobile phones or portable telephone devices), portable game machines, portable information terminals, audio playback devices, large game machines such as pin-ball machines, and the like.

In the case of having flexibility, the electronic device or lighting device of one embodiment of the present invention can be incorporated along a curved inside/outside wall surface of a house or a building or a curved interior/exterior surface of a car.

Furthermore, the electronic device of one embodiment of the present invention may include a secondary battery. It is preferable that the secondary battery be capable of being charged by non-contact power transmission.

Examples of the secondary battery include a lithium ion secondary battery such as a lithium polymer battery using a gel electrolyte (lithium ion polymer battery), a lithium-ion battery, a nickel-hydride battery, a nickel-cadmium battery, an organic radical battery, a lead storage battery, an air secondary battery, a nickel-zinc battery, and a silver-zinc battery.

The electronic device of one embodiment of the present invention may include an antenna. When a signal is received by the antenna, the electronic device can display an image, data, or the like on a display portion. When the electronic device includes a secondary battery, the antenna may be used for non-contact power transmission.

FIG. 51A illustrates a portable game machine including a housing 7101, a housing 7102, display portions 7103 and 7104, a microphone 7105, speakers 7106, an operation key 7107, a stylus 7108, and the like. The semiconductor device of one embodiment of the present invention can be used for an integrated circuit, a CPU, or the like incorporated in the housing 7101. When a normally-off CPU is used as the CPU, power consumption can be reduced, allowing a user to enjoy playing a game for longer than before. When the light-emitting device of one embodiment of the present invention is used as the display portion 7103 or 7104, it is possible to provide a user-friendly portable game machine with quality that hardly deteriorates. Although the portable game machine illustrated in FIG. 51A includes two display portions, the display portions 7103 and 7104, the number of display portions included in the portable game machine is not limited to two.

FIG. 51B illustrates a smart watch including a housing 7302, a display portion 7304, operation buttons 7311 and 7312, a connection terminal 7313, a band 7321, a clasp 7322, and the like. The semiconductor device of one embodiment of the present invention can be used for a memory, a CPU, or the like incorporated in the housing 7302. Note that when the display is a reflective liquid crystal panel and the CPU is a normally-off CPU in FIG. 51B, power consumption can be reduced, leading to a reduction in the number of times of daily charging.

FIG. 51C illustrates a portable information terminal including a display portion 7502 incorporated in a housing 7501, operation buttons 7503, an external connection port 7504, a speaker 7505, a microphone 7506, a display portion 7502, and the like. The semiconductor device of one embodiment of the present invention can be used for a mobile memory, a CPU, or the like incorporated in the housing 7501. Note that when a normally-off CPU is used, the number of times of charging can be reduced. The display portion 7502 is small- or medium-sized but can perform full high vision, 4K or 8K display because it has greatly high resolution; therefore, a significantly clear image can be obtained.

FIG. 51D illustrates a video camera including a first housing 7701, a second housing 7702, a display portion 7703, operation keys 7704, a lens 7705, a joint 7706, and the like. The operation keys 7704 and the lens 7705 are provided for the first housing 7701, and the display portion 7703 is provided for the second housing 7702. The first housing 7701 and the second housing 7702 are connected to each other with the joint 7706, and the angle between the first housing 7701 and the second housing 7702 can be changed with the joint 7706. Images displayed on the display portion 7703 may be switched in accordance with the angle at the joint 7706 between the first housing 7701 and the second housing 7702. The imaging device of one embodiment of the present invention can be used in a portion corresponding to a focus of the lens 7705. The semiconductor device of one embodiment of the present invention can be used for an integrated circuit, a CPU, or the like incorporated in the first housing 7701.

FIG. 51E illustrates a digital signage including a display portion 7902 provided on a utility pole 7901. The semiconductor device of one embodiment of the present invention can be used for a display panel of the display portion 7902 and an incorporated control circuit.

FIG. 52A illustrates a notebook personal computer including a housing 8121, a display portion 8122, a keyboard 8123, a pointing device 8124, and the like. The semiconductor device of one embodiment of the present invention can be used for a CPU, a memory, or the like incorporated in the housing 8121. Note that the display portion 8122 is small- or medium-sized but can perform 8K display because it has greatly high resolution; therefore, a significantly clear image can be obtained.

FIG. 52B is an external view of an automobile 9700. FIG. 52C illustrates a driver's seat of the automobile 9700. The automobile 9700 includes a car body 9701, wheels 9702, a dashboard 9703, lights 9704, and the like. The semiconductor device of one embodiment of the present invention can be used in a display portion and a control integrated circuit of the automobile 9700. For example, the semiconductor device of one embodiment of the present invention can be used in display portions 9710 to 9715 illustrated in FIG. 52C.

The display portions 9710 and 9711 are display devices or input/output devices provided in an automobile windshield. The display device or input/output device of one embodiment of the present invention can be a see-through display device or input/output device, through which the opposite side can be seen, by using a light-transmitting conductive material for its electrodes. Such a see-through display device or input/output device does not hinder driver's vision during the driving of the automobile 9700. Therefore, the display device or input/output device of one embodiment of the present invention can be provided in the windshield of the automobile 9700. Note that in the case where a transistor or the like for driving the display device or input/output device is provided in the display device or input/output device, a transistor having light-transmitting properties, such as an organic transistor using an organic semiconductor material or a transistor using an oxide semiconductor, is preferably used.

The display portion 9712 is a display device provided on a pillar portion. For example, the display portion 9712 can compensate for the view hindered by the pillar portion by showing an image taken by an imaging unit provided on the car body. The display portion 9713 is a display device provided on a dashboard portion. For example, the display portion 9713 can compensate for the view hindered by the dashboard portion by showing an image taken by an imaging unit provided on the car body. That is, showing an image taken by an imaging unit provided on the outside of the car body leads to elimination of blind areas and enhancement of safety. In addition, showing an image so as to compensate for the area which a driver cannot see makes it possible for the driver to confirm safety easily and comfortably.

FIG. 52D illustrates the inside of a car in which a bench seat is used as a driver seat and a front passenger seat. A display portion 9721 is a display device or an input/output device provided in a door portion. For example, the display portion 9721 can compensate for the view hindered by the door portion by showing an image taken by an imaging unit provided on the car body. A display portion 9722 is a display device provided in a steering wheel. A display portion 9723 is a display device provided in the middle of a seating face of the bench seat. Note that the display device can be used as a seat heater by providing the display device on the seating face or backrest and by using heat generated by the display device as a heat source.

The display portions 9714, 9715, and 9722 can display a variety of kinds of information such as navigation data, a speedometer, a tachometer, a mileage, a fuel meter, a gearshift indicator, and air-condition setting. The content, layout, or the like of the display on the display portions can be changed freely by a user as appropriate. The information listed above can also be displayed on the display portions 9710 to 9713, 9721, and 9723. The display portions 9710 to 9715 and 9721 to 9723 can also be used as lighting devices. The display portions 9710 to 9715 and 9721 to 9723 can also be used as heating devices.

FIG. 53A is an external view of a camera 8000. The camera 8000 includes a housing 8001, a display portion 8002, an operation button 8003, a shutter button 8004, a connection portion 8005, and the like. A lens 8006 can be put on the camera 8000.

The connection portion 8005 includes an electrode to connect a finder 8100, which is described below, a stroboscope, or the like.

Although the lens 8006 of the camera 8000 here is detachable from the housing 8001 for replacement, the lens 8006 may be included in a housing.

Images can be taken at the press of the shutter button 8004. In addition, images can be taken at the touch of the display portion 8002 which serves as a touch panel.

The display device or input/output device of one embodiment of the present invention can be used in the display portion 8002.

FIG. 53B shows the camera 8000 with the finder 8100 connected.

The finder 8100 includes a housing 8101, a display portion 8102, a button 8103, and the like.

The housing 8101 includes a connection portion for engagement with the connection portion 8005 of the camera 8000 so that the finder 8100 can be connected to the camera 8000. The connection portion includes an electrode, and an image or the like received from the camera 8000 through the electrode can be displayed on the display portion 8102.

The button 8103 has a function of a power button, and the display portion 8102 can be turned on and off with the button 8103.

The semiconductor device of one embodiment of the present invention can be used for an integrated circuit and an image sensor included in the housing 8101.

Although the camera 8000 and the finder 8100 are separate and detachable electronic devices in FIGS. 53A and 53B, the housing 8001 of the camera 8000 may include a finder having the display device or input/output device of one embodiment of the present invention.

FIG. 53C is an external view of a head-mounted display 8200.

The head-mounted display 8200 includes a mounting portion 8201, a lens 8202, a main body 8203, a display portion 8204, a cable 8205, and the like. The mounting portion 8201 includes a battery 8206.

Power is supplied from the battery 8206 to the main body 8203 through the cable 8205. The main body 8203 includes a wireless receiver or the like to receive video data, such as image data, and display it on the display portion 8204. The movement of the eyeball and the eyelid of a user is captured by a camera in the main body 8203 and then coordinates of the points the user looks at are calculated using the captured data to utilize the eye point of the user as an input means.

The mounting portion 8201 may include a plurality of electrodes so as to be in contact with the user. The main body 8203 may be configured to sense current flowing through the electrodes with the movement of the user's eyeball to recognize the direction of his or her eyes. The main body 8203 may be configured to sense current flowing through the electrodes to monitor the user's pulse. The mounting portion 8201 may include sensors, such as a temperature sensor, a pressure sensor, or an acceleration sensor, so that the user's biological information can be displayed on the display portion 8204. The main body 8203 may be configured to sense the movement of the user's bead or the like to move an image displayed on the display portion 8204 in synchronization with the movement of the user's head or the like.

The semiconductor device of one embodiment of the present invention can be used for an integrated circuit included in the main body 8203.

At least part of this embodiment can be implemented in combination with any of the embodiments described in this specification as appropriate.

Embodiment 12

In this embodiment, application examples of an RF tag using the semiconductor device of one embodiment of the present invention are described with reference to FIGS. 54A to 54F.

<Application Examples of RF Tag>

The RF tag is widely used and can be provided for, for example, products such as bills, coins, securities, bearer bonds, documents (e.g., driver's licenses or resident's cards, see FIG. 54A), vehicles (e.g., bicycles, see FIG. 54B), packaging containers (e.g., wrapping paper or bottles, see FIG. 54C), recording media (e.g., DVD or video tapes, see FIG. 54D), personal belongings (e.g., bags or glasses), foods, plants, animals, human bodies, clothing, household goods, medical supplies such as medicine and chemicals, and electronic devices (e.g., liquid crystal display devices, EL display devices, television sets, or mobile phones), or tags on products (see FIGS. 54E and 54F).

An RF tag 4000 of one embodiment of the present invention is fixed to a product by being attached to a surface thereof or embedded therein. For example, the RF tag 4000 is fixed to each product by being embedded in paper of a book, or embedded in an organic resin of a package. Since the RF tag 4000 of one embodiment of the present invention can be reduced in size, thickness, and weight, it can be fixed to a product without spoiling the design of the product. Furthermore, bills, coins, securities, bearer bonds, documents, or the like can have an identification function by being provided with the RF tag 4000 of one embodiment of the present invention, and the identification function can be utilized to prevent counterfeiting. Moreover, the efficiency of a system such as an inspection system can be improved by providing the RF tag of one embodiment of the present invention for packaging containers, recording media, personal belongings, foods, clothing, household goods, electronic devices, or the like. Vehicles can also have higher security against theft or the like by being provided with the RF tag of one embodiment of the present invention.

As described above, by using the RF tag including the semiconductor device of one embodiment of the present invention for each application described in this embodiment, power for operation such as writing or reading of data can be reduced, which results in an increase in the maximum communication distance. Moreover, data can be held for an extremely long period even in the state where power is not supplied; thus, the RF tag can be favorably used for application in which data is not frequently written or read.

Note that this embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

Example 1

In this example, a sample A and a sample B were fabricated using one embodiment of the present invention. Cross sections of the samples were observed and elementary analyses thereof were performed. The results are described.

FIG. 56 is a schematic cross-sectional view of the evaluation samples. The sample A and the sample B each include the substrate 100, the insulating layer 110, the oxide semiconductor layer 122, the conductive layer 166, the conductive layer 168, and an antioxidant film 178.

A silicon substrate was used as the substrate 100.

As the insulating layer 110, a 100-nm-thick silicon oxide film was formed by a thermal oxidation method. The insulating layer 110 was formed by thermal oxidation treatment at 950° C. in an oxygen atmosphere containing 3% chlorine.

The oxide semiconductor layer 122 was deposited to a thickness of 50 nm by a sputtering method using a target of In:Ga:Zn=4:2:4.1. The deposition conditions were as follows: the pressure in a chamber was 0.7 Pa; a DC power source was used and the power was 0.5 kW; the sputtering gas flow rates of an Ar gas and an oxygen gas were 30 sccm and 15 sccm, respectively; the distance between the sample and the target was 60 mm; and the substrate heating temperature was 200° C.

As the conductive layer 166, a 10-nm-thick cobalt film was formed by a sputtering method. The deposition conditions were as follows: the pressure in a chamber was 0.8 Pa; a DC power source was used and the power was 0.5 kW; the flow rate of an Ar gas as a sputtering gas was 50 sccm; the distance between the sample and the target was 150 mm; and the substrate heating temperature was room temperature.

As the conductive layer 168, a 20-nm-thick titanium film was deposited by a sputtering method. The deposition conditions were as follows: the pressure in a chamber was 0.1 Pa; a DC power source was used and the power was 12 kW; the flow rate of an Ar gas as a sputtering gas was 20 sccm; the distance between the sample and the target was 400 mm; and the substrate heating temperature was room temperature.

As the antioxidant film 178, a 20-nm-thick titanium nitride film was deposited by a sputtering method.

Next, heat treatment was performed on the samples. The heat treatment was performed using an RTA apparatus. The heat treatment was performed in two conditions. The sample A was subjected to a heat treatment at 300° C. in a nitrogen atmosphere for 60 seconds; and the sample B was subjected to a heat treatment at 500° C. in a nitrogen atmosphere for 60 seconds.

The cross sections of the samples were observed with a scanning transmittance electron microscope (STEM). As an apparatus for observation, RD-2300 manufactured by Hitachi High-Technologies Corporation was used. Elemental analysis was performed using energy dispersive X-ray spectroscopy (EDX). As an analyzer, Genesis 4000 R-TEM series manufactured by EDAX Inc. was used.

FIGS. 57A and 57B each show an image in which an observation photograph of the cross-section of the sample after the heat treatment overlaps with a line mapping analysis result thereof by EDX. In the line mapping analysis results, a portion difficult to see is indicated by dotted lines.

FIG. 57A is the analysis result of the sample A. FIG. 57A shows that the diffusion of elements in layers is not observed when the heat treatment is performed at 300° C. for 60 seconds. FIG. 57B is the analysis result of the sample B. In FIG. 57B, however, by performing heat treatment at 500° C. for 60 seconds, indium and gallium are diffused in the direction in which titanium exists and cobalt is diffused in the direction in which indium, gallium, and zinc which are constituent elements of the oxide semiconductor layer 122 exist. In addition, FIG. 57B also indicates that titanium which is a constituent element of the conductive layer 168 is diffused.

In the case where titanium was not used in the conductive layer 168, cobalt, indium, and zinc were not diffused even when heat treatment at 500° C. for 60 seconds was performed, although it is not shown in the drawing.

Therefore, it is found that one embodiment of the present invention promotes diffusion of constituent elements of the oxide semiconductor layer 122, the conductive layer 166, and the conductive layer 168, whereby an alloy is formed.

This application is based on Japanese Patent Application serial no. 2015-153217 filed with Japan Patent Office on Aug. 3, 2015, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A method for manufacturing a semiconductor device, comprising the steps of: depositing a first insulating layer over a substrate; depositing a first oxide insulating layer over the first insulating layer; depositing a first oxide semiconductor layer over the first oxide insulating layer; forming a second oxide insulating layer and a second oxide semiconductor layer by etching the first oxide insulating layer and the first oxide semiconductor layer with a first mask; depositing a third oxide insulating layer over the second oxide semiconductor layer and the first insulating layer; depositing a second insulating layer over the third oxide insulating layer; depositing a first conductive layer over the second insulating layer; forming a gate electrode layer and a gate insulating layer by etching the first conductive layer and the second insulating layer with a second mask; depositing a third insulating layer over the gate electrode layer, the second oxide semiconductor layer, and the first insulating layer; forming a sidewall insulating layer comprising a region in contact with a side surface of the gate electrode layer by performing dry etching on the third insulating layer; depositing a second conductive layer over the gate electrode layer, the sidewall insulating layer, the second oxide semiconductor layer, and the first insulating layer; depositing a third conductive layer over the second conductive layer; and forming a low-resistance region in the second oxide semiconductor layer by performing heat treatment, wherein the second conductive layer comprises a first element, wherein the third conductive layer comprises a second element, wherein the second oxide semiconductor layer comprises a third element, and wherein the low-resistance region comprises the first element, the second element and the third element.
 2. The method for manufacturing the semiconductor device according to claim 1, wherein the first element is any one of cobalt, nickel, platinum, vanadium, chromium, and palladium, and wherein the second element is any one of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, rhodium, iridium, and cobalt.
 3. The method for manufacturing the semiconductor device according to claim 1, wherein the low-resistance region comprises a region comprising an alloy.
 4. The method for manufacturing the semiconductor device according to claim 1, wherein the heat treatment is performed at a temperature higher than or equal to 450° C. and lower than or equal to 650° C.
 5. A method for manufacturing a semiconductor device, comprising the steps of: depositing a first insulating layer over a substrate; depositing a first oxide insulating layer over the first insulating layer; depositing a first oxide semiconductor layer over the first oxide insulating layer; forming a second oxide insulating layer and a second oxide semiconductor layer by etching the first oxide insulating layer and the first oxide semiconductor layer with a first mask; depositing a third oxide insulating layer over the second oxide semiconductor layer and the first insulating layer; depositing a second insulating layer over the third oxide insulating layer; forming a third insulating layer by performing planarization treatment on the second oxide insulating layer; forming a fourth insulating layer with a groove portion that reaches the third oxide insulating layer by etching part of the third insulating layer with a second mask; depositing a fifth insulating layer over the fourth insulating layer and the third oxide insulating layer; depositing a first conductive layer over the fifth insulating layer; forming a gate electrode layer and a sixth insulating layer by performing planarization treatment on the first conductive layer and the fifth insulating layer until the fourth insulating layer is exposed; forming a gate insulating layer by etching the fourth insulating layer and the sixth insulating layer with the gate electrode layer as a mask; depositing a seventh insulating layer over the gate electrode layer, the second oxide semiconductor layer, and the first insulating layer; forming a sidewall insulating layer comprising a region in contact with a side surface of the gate electrode layer by performing dry etching on the seventh insulating layer; depositing a second conductive layer over the gate electrode layer, the sidewall insulating layer, the second oxide semiconductor layer, and the first insulating layer; depositing a third conductive layer over the second conductive layer; and forming a low-resistance region in the second oxide semiconductor layer by performing heat treatment, wherein the second conductive layer comprises a first element, wherein the third conductive layer comprises a second element, wherein the second oxide semiconductor layer comprises a third element, and wherein the low-resistance region comprises the first element, the second element and the third element.
 6. The method for manufacturing the semiconductor device according to claim 5, wherein the first element is any one of cobalt, nickel, platinum, vanadium, chromium, and palladium, and wherein the second element is any one of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, rhodium, iridium, and cobalt.
 7. The method for manufacturing the semiconductor device according to claim 5, wherein the low-resistance region comprises a region comprising an alloy.
 8. The method for manufacturing the semiconductor device according to claim 5, wherein the heat treatment is performed at a temperature higher than or equal to 450° C. and lower than or equal to 650° C.
 9. A method for manufacturing a semiconductor device, comprising the steps of: forming an oxide semiconductor layer over a substrate; forming a gate electrode layer and a gate insulating layer over the oxide semiconductor layer; forming a sidewall insulating layer comprising a region in contact with a side surface of the gate electrode layer; depositing a first conductive layer over the gate electrode layer, the sidewall insulating layer, and the oxide semiconductor layer; depositing a second conductive layer over the first conductive layer; and forming a low-resistance region in the oxide semiconductor layer by performing heat treatment, wherein the first conductive layer comprises a first element, wherein the second conductive layer comprises a second element, wherein the oxide semiconductor layer comprises a third element, and wherein the low-resistance region comprises the first element, the second element and the third element.
 10. The method for manufacturing the semiconductor device according to claim 9, wherein the first element is any one of cobalt, nickel, platinum, vanadium, chromium, and palladium, and wherein the second element is any one of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, rhodium, iridium, and cobalt.
 11. The method for manufacturing the semiconductor device according to claim 9, wherein the low-resistance region comprises a region comprising an alloy.
 12. The method for manufacturing the semiconductor device according to claim 9, wherein the heat treatment is performed at a temperature higher than or equal to 450° C. and lower than or equal to 650° C. 